Chapter 12 Plant Kingdom Part 3 by Teaching Care online coaching classes

Chapter 12 Plant Kingdom Part 3 by Teaching Care online coaching classes

File name : Chapter-12-Plant-Kingdom-Part-3.pdf

 Pteris, Dryopteris and Pteridium (Fern).

Systematic position

Kingdom          –        Plantae

Sub kingdom     –        Embryophyta Phylum            –        Tracheophyta

Class               –        Leptosparangiopsida

Order               –        Filicales

Family             –        Polypodiaceae

Genus              –        Pteris, Dryopteris, Pteridium (For Rajasthan PMT & UP CPMT Students Only)

  • Habitat : Ferns live in moist, cool and shady places. They are perennial and evergreen. Fern Dryopteris filix-mas is commonly known as Beech fern or Male shield fern or Hay

 

scented fern. There are about 150 sps. and 25 sps. have been reported in India. It is found in sub-tropical regions as well as warm temperate regions.

(2)  Structure

  • External structure : Fern plant is sporophytic (2n) with an underground rhizomatous stem, large aerial leaves or fronds and adventitious Rhizome is sparingly branched in Dryopteris, moderately branched in Pteris and Adiantum and profusely branched in Pteridium. Young leaves show circinate ptyxis. Younger parts of leaves and rhizome are surrounded by brown hairy structures called scales or ramenta. Leaf bases are persistent. Leaves are pinnately compound unipinnately Pteris

Rachis

 

Pinnae

 

Young leaf with circinate ptyxis

 

 

 

Adv. roots

 

 

Petiole Persistent

leaf bases      Sori    Vein

 

 

Rhizome

 

  • (B)

 

vittata, incompletely bipinnate Dryopteris filix mas, bipinnate in Pteris, biaurita, Dryopteris rigida, tripinnate in Pteridium aquilinum. The leaves show open dichotomous type of venation.

(ii)  Internal structure

Fig : Dryopteris (A) Plant showing habit

  • Fertile pinnule

 

Epiblema

 

  • Root : The epiblema is provided with unicelled root hair. It followed by thin walled outer cortex. The inner cortex is thick walled (lignified). The cortex is followed by endodermis with characteristic casparian strip. The pericycle may be 1-2 cells thick. It is also made up of thin walled The xylem is diarch and exarch with two phloem groups alternating the protoxylem.
  • Rhizome : It Pteridium aquilinum the epidermis is followed by sclerenchymatous hypodermis and the thin walled ground The young portion has a siphonostele, but this later forms two concentric cylinders of vascular tissue. The outer ring corresponds to a dictyostele (e.g. Dryopteris and Pteris) or dicyclic or polycylic dictyostele

Outer cortex

 

 

 

Inner cortex

 

 

Endodermis Pericycle Metaxylem

Phloem

Protoxylem

 

 

Fig : T.S. of Dryopteris root

 

 

 

 

(e.g., Pteridium) which is distinguishable into small meristeles. The inner ring comprises generally two medullary meristeles. The two rings are separated by two patches of thick walled tissue. Each meristele has its own endodermis followed by thin walled pericycle. It is amphicribal (ectophloic) with mesarch xylem.

The phloem lacks companion cells. In Dryopteris filix-mas the vasculature comprises a dictyostele consisting of a ring of meristele. In Pteris, however, the vasculature ranges from solenostele to polycyclic dictyostele.

Rachis : The epidermis is followed by thick walled hypodermis. The ground tissue is thin walled. In Pteridium aquilinum several meristeles lie irregularly scattered in the ground tissue. In Dryopteris filix-mas there are 6–8 meristeles arragned in a horse-shoe like manner and single arched with hooked xylem in Pteris. The structure of the meristele is similar to that of rhizome.

(d) Leaf lamina : The pinnule of Pteridium aquilinum shows distinct upper and lower epidermis. The lower epidermis is provided with stomata. The mesophyll is differentiated into an upper zone of palisade parenchyma and a lower of spongy paranchyma. The spongy parenchyma has large intercellular spaces. The vascular strands lie embedded in mesophyll. Each strand is generally amphicribal with its own endodermis and pericycle but sometimes they are bicollateral also. The bundles in minor veins are collateral.

 

 

Upper Epidermis

Phloem

Xylem

Glandular cell

 

Fig : Dryopteris (A) T.S. rachis (basal portion)

  • S. rachis (Upper portion)

Lower epidermis

Stoma         Bundle sheath  Air space

Fig : V.T.S. of pinnule or sterile leaflet (leaf lamina) of Dryopteris

 

 

(3)  Reproduction

  • Vegetative reproduction : Vegetative reproduction can occur through fragmentation or rhizome and adventitious buds and these on separation

 

gives rise to new fern plant.

  • Sexual reproduction : Sexual reproduction takes place through Spores are born in sporangia. The spores are of one kind only (homosporous). When leaves are mature they bear groups of sori on the under surface of fertile pinnae. Such fronds are called sporophylls. The sori are borne in two rows on two sides of median vein of pinnule in between the margin and the midrib.

Sori are linear and submarginal in Pteris

 

 

 

Sori (a)

Upper epidermis

Mesophyll      Vein

Lower epidermis Spores (haploid)

Sporangia (stalked)

 

Covered indused (membranous) (true indusium)

Placenta

(b)

Fig : Dryopteris (a) Part of sporophyll with sori

(b) T.S. of sorus

 

 

 

 

and Pteridium and median abaxial in Dryopteris. Each sorus is surrounded by a kidney-shaped covering called indusium. In Dryopteris, true indusium is present because this arises from placenta or placental tissue, from which sporangia arise. (In Pteris false indusium is there because it is formed by leaf margins).

In sorus of Pteridium is covered by two flap like appendages that protect the sporangia. The upper flap is called false indusium and lower is called the true indusium. In the centre of sorus, the vein ends into placental tissue from where arise a number of sporangia. The sorus is mixed in Dryopteris (i.e., no definite arrangement of sporangia).

Sporangium : The sporangial development is leptosporangiate i.e., it develops from a single superficial initial. (In eusporangiate type the sporangium arises from a group of initials.)

A sporangium is distinguishable into a stalk and a capsule. The stalk is multicelled and biseriate. The capsule is oval or elliptical and biconvex in

 

shape. It consists of a single layered wall followed by double layered tapetum that encloses the archesporium. The archesporial cells divide and redivide to form a mass of sporogenous tissue. Most of the sporogenous cells behave a spore mother cells. They undergo meiosis to form tetrahedral tetrads of (haploid) spores. As a result 32–64 spores are formed in each capsule. The tapetal layer is nutritive. It degenerates at maturity of the sporangium.

Stomium

 

 

 

Spore

 

 

 

Spores Stalk

 

At the capsule matures, about four lower median cells of the jacket

Fig : Dryopteris – One sporangium

 

stretch tangentially. Of these, two median ones identify the place from where the capsule opens. This is called stomium. The remaining cells of the same median row of the jacket covering about three fourths of the perimeter become specialised.

They develop a thickening along their radial and inner tangential walls. This layer is called annulus. At maturity the inducium dries exposing the sorus. The cells of the annulus loose water. Due to presence of thickening along the radial and inner tangential walls, their upper walls contract and the inner ones straight and the annulus coils. Thus, it exerts pressure on the wall resulting in breaking of the capsule between the cells of stomium thereby releasing the spores.

Gametophytic generation

  • Spores :  It is the first cell of gametophytic

 

generation. Spores are double layered. The outer wall exospore is much thicker than inner endospore. On approach of favourable condition spore germinates to form a filamentous gametophyte which develops into green and heart shaped prothallus.

  • Prothallus : It lies flat on the soil surface, attached by means of numerous delicate rhizoids. The fern prothallus is single celled thick. Although at maturity, the portion below the notch becomes many celled thick, i.e. cushion. Prothallus shows polarity and The dorsal surface is smooth and the ventral is provided with

Apical notch Cushion

Archegonia (upper side and neck points downwards)

 

Antheridla (lower side)

 

Rhizoids (unicellular)

 

Fig : Mature prothallus of Dryopteris

 

 unicelled rhizoids and sex organs. Diameter of fern prothallus is 5 or 6 mm to 13 mm and each cell of prothallus is having single nucleus and many discoid chloroplasts.

Fern prothallus is monoecious but protandrous (antheridia mature first). Antheridia are present in between the rhizoids while archegonia are present near the apical notch.

  • Antheridium : It consists of a three celled jacket enclosing a mass of androgonial cells. The two lower jacket cells are ring like (first and second ring cells), and the terminal cell is called as opercular cell or cover cell or cap cell. Sometimes, there may be two cap cells and in that case the jacket is four The last generation of androgonial cells forms the androcytes. There may be 20–25 androcytes in an antheridium, each of which is metamorphose into a spirally coiled, multiflagellate antherozoid.

 

Spermatocyte

 

Fig : Dryopteris (a) L.S. antheridium (b) dehisced antheridium

  • Multiflagellate spermatozoids

 

 

  • Archegonium : It is a flask-shaped structure having venter and neck. Neck is projected out of the prothallus and is curved posteriorly. Venter is having basal large egg cell and upper small venter canal cell. The neck is having single neck canal cell but is binucleate. Venter is not having any covering or jacket but neck is surrounded by jacket of 4 ventrical rows of

 

Prothallus cells

Oosphere

 

 

Neck

 

 

 

(a)

Ventral canal cell

Binuclete neck canal cell

 

 

 

 

Mucilage (b)

 

Fig : Dryopteris (a) Mature archegonium

  • Before fertilization

 

 

 

 

  • Fertilization and development of sporophyte : Before fertilization the walls of androgonial cells disorganise to form a mucilagenous mass. The opercular cell is removed when it comes in contact with external The neck canal cell and the ventral canal cell degenerate. The cover cells split apart giving a free passage to incoming antherozoids. The antherozoids are attracted by chemotactic stimulus which is probably provided in the form of malic acid. A single antherozoid is able to fuse with egg to form zygote (2n), which is beginning of sporophytic generation. Zygote divides first by vertical division, followed by another vertical division and quadrat stage is formed. Then octant stage is formed by transverse division. The foot and root develop from four hypobasal cells and cotyledons as well as stem develop from epibasal cells and thus sporophytic plant is formed. At maturity foot is hemispherical mass of gametophyte from which it absorbs food. Generally a single sporophytic plant develops from single gametophyte or prothallus. The fern sporophyte is initially dependent upon gametophyte but later on becomes independent.

The life cycle is diplohaplontic with heteromorphic alternation of generation.

Adventitious bud

Fragmentation

 

Vegetative reproduction

 

 

 

 

Embryo Oospore

Fern plant

 

 

Sporophytic generation

 

Sporophyll

Sorus

 

 

Fertilization

 

Oosphere Sperm

Archegonium

diplophase (2n)

 

 

Gametophytic generation haplophase (n)

Sporangium

Spore mother cells

 

Meiosis

 

Antheridium                                      Spores

 

Prothallus

Fig : Graphical representation of life cycle of fern

 

 Selaginella.

Systematic position

Kingdom          –        Plantae

Sub kingdom     –        Embryaphyta Phylum            –        Tracheophyta

Class               –        Ligulopsida

Order               –        Selaginellases

Family             –        Selaginellaceae

Genus              –        Selaginella

 

  • Habitat : Selaginella is commonly called the little club moss or spike moss and is having about 760 species, out of which 70 species have been reported from India. Selaginella is mainly found in damp shaded places. A few species are xerophytic and can withstand the dry conditions for months together. In dry conditions, the plant rolls up into a compact ball and root system is disorganized. During the rainy conditions the ball on absorbing moisture, becomes green again. Such plants are called resurrection plants or bird’s nest moss, g., S.lepidophylla, S.bryopteris (Sanjeevani) and S.rupestris (ornamental).

The epiphytic species grow on the branches and trunks of moss covered trees. The common epiphytic species are S. chrysocaulos, S. kraussiana, S. oregana, S. chrysorrhizus.

(2)  Structure

  • External structure : The plant body is sporophytic (2n), which is an evergreen and delicate herb having adventitious roots. The plants show great variation in their morphology. Some species are prostrate growing upon the surface (g., S.kraussiana), some are suberect (e.g., S.trachphylla) and others are climbers (e.g., S.allegans). Plants are many meters long in S.willdenovii and only few centimetre long in S.spinulosa. The stem is covered with four rows of small leaves, out of these two rows are of smaller

leaves and two of large leaves species with

 

dimorphic leaves such as S.kraussiana, S.helvetica, S.lepidophylla, S.chrysocaulos etc. are grouped in

Fig : Selaginella kraussiana : (A) General habit (B) A part of the plant

  • Small portion of (B) showing arrangement of leaves

 

subgenus heterophyllum. Species having leaves uniform in size are grouped in the subgenus homeophyllum. These species are S.spinulosa, S.rupestris, S.pygmaea and S.oregana etc.

Leaves are sessile, ovate or lanceolate with acute apex. Unbranched midrib is present in the centre of each leaf. The leaves are ligulate, i.e., a flap-like outgrowth is present at the base on adaxial side called ligule. It may be fan-shaped or tongue-shaped or lobed or fringed. At the base of ligule, there is present a sheath of elongated cells called glossopodium (secretory).

The leaves possess a midrib but there is no venation. At the place of bicuraction of stem, a leafless, colourless, positively geotropic, elongated, cylindrical structure grows downwards. This is called the rhizophore and is quite different from the root in that it has no root cap. Rhizophores are not present in S.cuspidata. Rhizophores typically develops of adventitious roots at its apex.

(ii)  Internal structure

  • Root : The root is distinguishable into a single layered epidermis having root This is followed by a 3– 4 layered thick walled hypodermis representing outer cortex. The inner cortex is thin walled. The endodermis delimits the cortex. It is generally not distinct but in S.willdenovii it is very clear. The endodermis is followed by 1–3 layered pericycle. The stele is a protostele. It has a central core of xylem surrounded by phloem which is horse shoe shaped. It has a single protoxylem element (monarch). The xylem is exarch.

 

 

  • Stem : The stem is internally distinguishable into a single layered epidermis having no stomata. This is followed by cortex. The outer cortex is thick walled (hypodermis) followed by thin walled inner cortex. The hypodermis is very well developed in xerophytic The stele is suspended by unicelled (rarely multicelled) trabaculae (modified endodermal cell). This layer, due to presence of casparian strips is regarded as endodermis. The stele is a protostele (haplostele) surrounded by a pericycle with a central core of xylem enclosed by phloem. Protostele is diarch and exarch.

 

 

Root hair

 

 

Epiblema

Cuticle

Epidermis

 

 

Cortex

Pericycle

 

 

Fig. T.S. root of Selaginella

Hypodermis Cortex Metaxylem

Pericycle Protoxylem

Endodermis

 

Phloem

 

Fig. T.S. stem of Selaginella

Protoxylem Metaxylem Phloem

Endodermal trabeculae

Casparian band

 

Xylem

 

  • Leaf : The leaf displays a simple structure. It is distinguishable into upper and lower epidermis enclosing mesophyll tissue in The upper and lower epidermal layers are 1-cell thick each. The lower epidermis is provided with stomata. The mesophyll is uniform, being composed of elongated chlorenchymatous cells with large intercellular spaces. Each measophyll cells has one (S.martensii), two

(S.kraussiana) or eight (S.willdenovii) chloroplasts. Each

 

 

Cuticle     Xylem       Chloroplasts

Upper epidermis

 

chloroplast  has  several  pyrenoid-like  bodies  similar  to

Lower epidermis

Stoma

Phloem

Air space

 

Anthocerotales. The single midrib bundle is concentric, amphicribal (ectophloic) with annular or spiral tracheids surrounded by a few sieve elements.

Bundle sheath

Fig : V.S. of leaf of Selaginella

 

  • Rhizophore : The anatomy of rhizophore is almost similar to root. The epidermis is cutinised and it is followed by cortex. The outer cortex is thickwalled (hypodermis) whereas inner one thin walled. The innermost layer of the cortex is endodermis which is followed by pericycle. The stele is a protostele. The xylem is exarch with several protoxylem groups. In kraussiana, centroxylic condition (having protoxylem in the centre surrounded by metaxylem elements) has been recorded.

 

 

 

  • Reproduction : Reproduction takes place by vegetative and sexual (by spores)
    • Vegetative reproduction : It is of rare occurrence and may takes place by following methods :
  • Fragmentation : It occurs during very humid Some branches act as adventitious branches, which get separated from the plant and give rise to new Selaginella plants, e.g., in S.rupestris.
  • By resting buds : In some cases, terminal leaves get overlapped and become fleshy and form resting buds, which are means of vegetative reproduction, g., in S.chrysocaulos.
  • By tubers : In chrysorrhizus, some branches penetrate into substratum and at terminal ends swell to form tubers, which give rise to new plants.
  • By apogamy : In some cases, development of sporophyte occurs directly from gametophyte without intervention of sex organs, it is called apogamy and such plants are genetically haploid.
    • Sexual reproduction : The reproductive structure in Selaginella is strobilus or spike. It is a sessile structure and develops at the terminal ends of the branches and its length varies from 1/4th of an inch to 2–3 inches in different

A strobilus is having many ligulate sporophylls arranged in cluster, each bearing a small, short, stalked sporangium on its upper surface. The sporangia are of two types :

  • Megasporangia : Borne on megasporophylls. Megasporangium is pale greenish and contains chalky white, yellow or orange megaspores. The megasporangium is four-lobed structure with a 2-layered jacket, one layer of tapetum and a large number of microspore mother cell. However, only one megaspore mother cell is After meiosis it produces 4 megaspores out of which 1–3 may degenerate. In S.rupestris, there is only a single megaspore.
  • Microsporangia : Borne on microsporophylls having a large number of small Thus Selaginella is heterosporous. Microsporangium is pale yellow, oval or

spherical body, with 2-layered jacket, one layered

tapetum and a number of microspore mother cells which undergo meiosis and form haploid microspores.

 

The main body consists of a wall having two layers, inside which are present numerous small microspores (400–2000). Development of sporangium is of eusporangiate type.

In most of the cases, the strobilus or spike bears two types of sporophylls; the lower are megasporophylls and the upper ones are

Microsporangium

Microsporophyll Megasporangium

Megasporophyll

 

 

 

 

Megaspores Megasporangium

Megasporophyll

Microspores

Microsporangium Microsporophyll

 

 

Central axis

 

 

Ligule

 

microsporophylls. In S.kraussiana there is single megasporophyll at the base of spike and the rest of upper are microsporophylls.

Fig : Selaginella : (a) A strobilus showing compactly arranged

sporophylls (b) L.S. through strobilus

 

In some cases strobilus contains either micro or megasporophyll, i.e., in S.gracilis and S.astrovirdis, while S. martensii and S.caulescens show intermixed micro and megasporophylls. Some sps. possess megasporangia on the ventral side and microsporangia on the dorsal side, e.g., S.oregana and S.inaequalifolia.

Wall                                                              Wall

  • (B)

Fig : Selaginella : (A) V.S. microsporangium (B) V.S. megasporangium

  • Mechanism of sporangial dehiscence : On maturation, the sporangium splits vertically from the upper end into two valves (vertical apical splitting). The lower cup-shaped portion shrinks and the spores come out through apical slits. This is brought about by cohesion owing to hygroscopic changes in the apical and lateral part of the sporangial This liberation of spores takes place at intervals in small masses.

The spores starts germinating inside the sporangium before their release; this is known as precocious germination. According to Goebel, the violent dispersal of spores is an adaptation for cross fertilization in that it helps to bring spores from different plants near each other. This is further proved by the protandrous nature of the strobilous.

  • Germination of microspore : The microspore is a double layered structure and contains oil droplets. The outer wall exospore is much thicker (spiny) than inner endospore. It measures 15–50m in diameter. The microspore on germination forms the male gametophyte. The structure and development of male gametophyte was first described by Slagg (1932). The first division leads to formation of a small prothallial cell and a large antheridial cell. The larger antheridial cell, by further divisions, gives rise to central group of four primary androgonial cells, surrounded by eight jacket cells. At this 13-celled stage (1 prothallial + 8 jacket cells and 4 primary androgonial cells), the microspore is shed from microsporangium. Each of the central groups of cells divides and redivides and finally forms about 256 spirally coiled antherozoids with two flagella (biflagellated); the jacket cells disintegrate. It takes about three weeks for germination of microspore and formation of antherozoids or
  • Germination of megaspore : The megaspore has three wall layers namely exospore, mesospore and endospore. It measures 1.5 – 5.0 mm in diameter. The megaspore on germination forms the female gametophyte. Generally the megaspore germinates inside the megasporangium (i.e., in situ). In some sps., megaspores are shed after the development of first archegonium, e., in S.kraussiana, while in S.apoda and S.rupestris, megaspores are not liberated till a well developed embryo is formed.

During the development of female gametophyte, the protoplasm after contraction forms a small sac-like structure. The outer wall bursts into two layers, the exospore and mesospore. At this stage, megaspore contains a haploid nucleus which by division produces many nuclei. Wall formation takes place in the upper beak-like portion

 

 

 

and a small-celled cellular tissue is formed. This is one celled thick at the sides and three celled thick in the middle. This is female prothallus. Some superficial cells at apex enlarge and act as archegonial initials and form the archegonia. The megaspore bursts exposing the female prothallus. Vestigial rhizoids develop.

Archegonium are sessile and embedded type and consists of very short neck having a single neck canal cell and a venter, having a single ventral canal cell and an egg.

  • Fertilization : Usually the male gametophytes are shed from the microsporangium on the ground at 13- celled stage. Here they complete their development ultimately producing spermatozoids. These are liberated by the decay of the microspore If the microspore falls near the mature female gametophyte, the sperms swim from the male gametophyte to reach archegonia and one sperm fuses with egg to form zygote. Water is necessary for fertilization and sperms are attracted due to malic acid.
  • Development of embryo or Sporophyte : The oosphere after fertilization gets surrounded by wall and become oospore. The oospore (zygote) divides transversely into two cells, the upper epibasal cell which forms suspensor cell and the hypobasal cell which develops into

The embryo differentiates into foot, root, primary stem with two rudimentary leaves and rhizophore. By growth of stem and the root, the young sporophyte becomes independent of the gametophyte tissue and falls on the ground where the primary rhizophore forms roots that grow into the soil and the plant starts independent life.

In some species of Selaginella, the archegonial initial develop apogamously into embryo. In S.intermedia, no microspores are formed. Here the embryo develops parthenogenetically from the egg. In S.helvetica, the archegonia fails to open and here also parthenogenetic development of embryo is seen.

There is distinct heteromorphic alternation of generations in Selaginella.

Fragmentation Bulbils Tubers

Vegetative reproduction

 

 

Embryo

Selaginella

 

Sporangiferous spike

 

 

Oospore

Sporophytic generation

Microsporophyll

 

Megasporophyll

 

 

Fertilization

diplophase (2n)

Microsporangium

Megasporangium

 

Microspore

 

SpermatozoidGametophytic

mother cell

Megaspore mother cell

 

Oosphere (egg)

generation

haplophase (n)

Microspore

Meiosis

 

Antheridium   Male

 

Archegonium

gametophyte

Megaspore

 

Female gametophyte

Fig :  Graphical representation of life cycle of Selaginella

 

 

 

 Pinus.

Systematic position

Kingdom          –        Plantae

Sub kingdom     –        Embryophyta Phylum            –        Tracheophyta

Class               –        Gymnospermae

Order               –        Coniferales

Family             –        Pinaceae

Genus              –        Pinus (For MP PMT Students Only)

  • Habitat : It is commonly known as pine with about 90 species among which six species are found in (N. East and N. West Himalayas) occurring in wild state. These are Pinus gerardiana (Chilgoza pine), P. Wallichiana (Blue pine or Kail), P.roxburghii (Chir pine), P.merkusii (Teenasserin pine), P.insularis (Khasi pine), and P.armandi (Armand’s pine). In addition to these, 4 sps. of exotic pines, i.e., P.montana, P.laricia and P.sylvestris (Scotch pine) and P.strobus (white pine) have been introduced in India. P.excelsa are found at maximum height i.e. grow upto 3500 m above see level.

(2)  Structure

  • External structure : Pinus is an evergreen, perennial plant of xerophytic nature. Mostly the species are tall and straight. The whorled branching gives a typical conical or excurrent appearance to the plant (due to apical dominance). The plant body is sporophyte and the plants

are monoecious. The plant body is differentiated into roots, stem and leaves.

  • Root : A prominent tap root is present which does not penetrate deep into the soil. Lateral roots which develops later, grow extensively and help in anchoring the plant in the soil. Root hairs are Ectotrophic mycorrhiza i.e. symbiotic association of some fungal hyphae with the ultimate branches of roots, is of common occurrence.

 

  • Stem : The stem is erect, thick, cylindrical and branched. The branching is monopodial The main stem is covered by scaly bark. Branches are developed from the buds present in the axil of scale leaves and appear to be in whorls. These branches develop every year and help in calculating the age of the plant.

Branches are of two types :

 

 

 

 

 

Scale leaves

Foilage leaves (needles)

 

 

Dwarf shoot

 

l  Long shoots  or  Branches  of  unlimited  growth  :

These have apical buds, grow indefinitely in whorls each

Long shoot

Scale leaves

 

year from the buds in the axil of scale leaves. These shoots spread out horizontally and bear scale leaves on them.

Fig : A part of Pinus stem showing two types of branches (long and dwarf) and two types of leaves(scale and foliage leaves)

 

  • Dwarf shoots or Branches of limited growth : These branches lack apical buds and grow for a definite or short They arise in the axil of scale leaves on long shoots.
  • Leaves : The leaves are of two types e., dimorphic – scale leaves and foliage leaves.
  • Scale leaves : The scale leaves are small membranous and brown. They are present on both types of branches (e. long and dwarf shoots). Scale leaves are non-photosynthetic. These protect the young buds.
  • Foliage leaves : The foliage leaves are green, needle like and are born at the tips of the dwarf shoots only. Their size and number is different in different species. The dwarf shoot with needles is called a spur. On the basis of number of needles, spur is of different types as :

Monofoliar (with one needle), e.g., P. monophylla.

Bifoliar (with two needles), e.g., P. merkusi and P. sylvestris. Trifoliar (with three needles), e.g. P. gerardiana and P. roxburghii. Pentafoliar (with five needles), e.g., P. wallichiana, P. occelsa.

(ii)  Internal structure

  • Root : The young root of Pinus is identical with the dicot A T.S. of root reveals the following structures.
  • Epiblema : It is the outermost layer of compactly arranged It gives out many thin and unicellular root hair.
  • Cortex : It is composed of many layered of thin walled parenchymatous
  • Endoermis : A single layer of suberized

 

  • Pericycle : Endodermis is followed by multilayered
  • Vascular tissues : Radial vascular bundles are

Xylem : Exarch condition with bifurcated (Y-shaped) protoxylem. Resin canal is present between two arms. Xylem is devoid of vessels.

Phloem : Alternating with the xylem groups are present phloem patches. Companion cells are absent.

Pith : Pith is generally absent. If present, it is very small

Root hair Epidermis

Pith

Resin canal

 

Protoxylem

 

Cortex

Endodermis Pericycle

Pholem

 

 

Resin cells Resin canal

Metaxylem

 

and made-up of parenchymatous cells.

Fig : T.S. of young root of Pinus

 

  • Secondary growth : In young roots, cambium is absent but at maturity below the phloem patches, arches of cambium are formed. It cuts off secondary xylem on the inner side and secondary phloem on the outer

The cells of the outermost layer of pericycle form cork cambium (phellogen), which cuts off phellem (cork) on the outer side and phelloderm (secondary cortex) on the inner side. Finally epiblema ruptures and the cork layer is exposed.

 

 

 

  • Stem : S. of a young shoot shows following tissue.
  • Epidermis : It is the outermost layer made up of small compactly arranged cells (heavily cuticularised).
  • Hypodermis : Below epidermis are 4–5 layers of sclerenchymatous cells constituting
  • Cortex : Inner to the hypodermis is a wide zone of cortex, some cells are filled with tanin.
  • Endodermis : It is the innermost layer of the cortex, made-up of single layer of

 

  • Vascular cylinder (Stele) : It is of eustelic type having a ring of 5–8 closely arranged vascular bundles. Vascular bundles are conjoint, collateral and

Xylem : It is endarch, consists of only tracheidal cells, vessels are absent. Therefore wood is known as non-porous.

Protoxylem consists of annular and spiral tracheids. Metaxylem

tracheids have uniseriate bordered pits on their radial walls. These are

 

Pith

Cortex

 

Resin canals Endodermis Pericycle

Phloem Cambium

Protoxylem

 

Metaxylem

 

also having bars of sanio.

Phloem : It is situated on the outer side of vascular bundle and is

made-up of phloem parenchyma and sieve cells. Companion cells are lacking.

Epidermis

 

Fig : T.S. of young stem of Pinus

 

Cambium : In between the xylem and phloem of each vascular bundle, there is a strip of intrafascicular cambium.

Medullary ray : In between the vascular bundles is a zone of parenchymatous cells connecting the pith and the cortex.

Pith : In the centre of the stem is a zone of thin-walled parenchymatous cells known as pith. Some of the pith cells are filled with resinous substances.

  • Secondary growth : Secondary growth is similar to that of dicot root. Wood is pycnoxylic and monoxylic. Vascular rays are linear (uniseriate) but fusiform (multiseriate) area of pasage of resin ducts. Some of the parenchymatous cells in between the adjacent vascular bundles form interfascicular cambium. Both inter and intrafascicular cambium form a complete ring of These cambium cells cut cells on the inner side forming secondary xylem and on outer side secondary phloem.

The ring of primary cambium remains active only for a year. The activity of the cambium stops in the winter season and again resumes in the following spring. The secondary xylem thus formed clearly shows a number of annual rings. Each annual ring consists of a zone of spring wood and autumn wood.

Autumn wood : It is formed during autumn season and the cells of this wood are smaller, squarish and thick.

Spring wood : It is formed during spring season. The cells of this wood are thinner, large and polygonal. The wood is termed as pycnoxylic (compact and hard).

 

 

 

  • Leaf (Needle) : The outline of foliage leaf varies according to the number of needles in the spur, e. in monofoliar spur of P. monophylla, it is circular, in bifoliar spur of P.sylvestris, it is semicircular and is triangular in trifolial spur of P. roxburghii. Internal structure of the needle is same in all species of Pinus. Needle shows xerophytic characters.
  • Epidermis : It has a single-layered, thick-walled epidermis, covered with thick cuticle and is interrupted by sunken stomata throughout the surface (amphistomatic).
  • Stomata : Each stoma has two guard cells and two subsidiary It opens outside into a cavity called

 

vestibule and inside into a substomatal cavity.

  • Hypodermis : Below the epidermis is present a few layered thick sclerenchymatous It helps in

mechanical support.

 

Ebistomatal cavity Subsidiary cell

Guard cell

 

 

 

Parenchymatous ray

Endodermis

Substomatal cavity

 

Stoma

Cuticle

Epidermis Hydodermis Resin canal

 

Albuminous cells Tracheidal cells

 

 

  • Mesophyll :      There   is   no differentiation                   into     palisade

Vascular bundle

Phloem Xylem

 

Mesophyll

Parenchymatous pericycle

Sclerenchymatous pericycle

 

and spongy parenchyma. The

Fig : T.S. neddle of Pinus roxburghii

 

cells of this region are thin-walled, parenchymatous, polygonal, compactly arranged, having chloroplasts and starch grains. Peg-like infoldings arise from the inner surface.

  • Vascular cylinder : It is surrounded by single-layered endodermis having barrel-shaped cells with casparian
  • Pericycle : Just below the endodermis is multilayered pericycle having a “T” shaped mass of sclerenchymatous cells between two vascular Transfusion tissue occurs on the side. Each bundle is collateral, open and endarch.

The needle of P.monophylla has a single vascular bundle whereas in P.roxburghii, the number is two.

  • Reproduction : Pinus reproduces only sexually. Pinus plant represents sporophytic generation. Unlike Cycas, here the micro and megasporophylls form compact male and female cone or strobilus respectively. The megasporangia are produced on ovuliferous scales formed along with a The ovuliferous scales and bracts constitute the female cone. The cones are generally monosporongiate. Rarely bisporangiate cones are formed in
  1. roxburghii, P. montana and P. maritima.
  • Male cone or Staminate strobilus : The male cones are borne in a cluster on a branch of unlimited growth behind the apical bud, in the axil of a scale leaf. A male cone is, thus, equivalent to a dwarf shoot. In a cluster, there may be 15–140 male cones. The male strobilus is an ovoid structure measuring 2 to 4 cms. in length and 0.5 to 0.7 cms. in diameter. A cone consists of a central axis bearing 60–135 microsporophylls in spiral It is, therefore, comparable to male flower of angiosperms.

 

 

 

  • Microsporophyll : The microsporophylls or ‘stamens’ are spirally arranged in a compact manner on the cone axis. The microsporophyll is a brown coloured triangular structure consisting of a short stalk or ‘filament’ and a leaf like flattened structure of ‘anther’. Each sporophyll is provided with two microsporangia on its abaxial surface. The terminal sterile portion of the sporophyll is turned upward to protect the upper sporangia. It is called apophysis. Some of the lower microsporophylls are sterile having no sporangia associated with

Micro sporophyll

 

Pollen grains

 

 

Sterlie portion

 

 

 

Micro sporangium

 

(B)

 

 

Microsporangia

 

 

 

  • (B)

Dehisced microsporangia

Microspores

 

Fig : (A) A young male cone of Pinus

  • Male cone in radial longitudinal section

Fig : Pinus (A) Ventral view of microsporophyll

(B) Ventral view of microsporophyll with dehisced microsporangia

 

  • Microsporangium : The microsporangia are sessile elongated, cylindrical, structures. The sporangial development is of eusporangiate type. Each sporangium consists of a 2–3 layered The inner most wall layer is called tapetum, which encloses a mass of sporogenous tissue. The sporogenous cells divide and re-divide and finally behave as microspore mother cells or pollen mother cells (PMS). The PMS undergo meiosis to form tetrahedral tetrads of microspores. The tapetum is a nutritive layer which degenerates at maturity of the anther.
  • Female cone : The female cone is an elongated, ovoid structure comprising a central cone axis on which the ovuliferous scales and bracts are spirally arranged in acropetal Usually the cone is 15 – 20 cms. long but in P. lambertiana they are 60 cms. long. The female cones take three years time to develop and mature.

The cones are produced in clusters of 1 to 4 from places where normally dwarf or spur shoots have developed. They arise in a group of 1 – 4 cones on a long shoot in the axil of a scale leaf in place of a dwarf shoot. In the first year, the female cone is reddish-green measuring about 1–2 cms in length having compactly arranged sporophylls. The second year cone is much larger, again with compact sporophylls. In the third year, the cone axis elongated and hence the sporophylls separate from each other.

  • Megasporophyll : Each megasporophyll is differentiated into two parts – Lower part is bract scale and upper part is ovuliferous scale.
  • Bract scales : These are small, dry, membranous structures attached with the cone axis These are also known as carpellary or cover scales. At the time of maturity, these bract scales become rolled up and thus help in the dispersal of seeds.

 

 

 

  • Ovuliferous scale : This is a woody, brownish structure borne on the dorsal side of the bract Each ovuliferous scale is triangular with narrow basal part and upper broader part in the form of disc, known as apophysis. The apophysis appears to be rhomboidal and possesses a small point known as umbo. On the dorsal side, near the base of each ovuliferous scale, are attached two ovules with their micropyles directed towards the cone axis. Florin gave the term seed scale complex to the bract scale along with associated ovuliferous scale.

 

 

 

Fig : Female cone of

Pinus

  • (B)

Fig : Pinus (A) Longitudinal section of female cone (B) A megasporophyll

 

 

  • Megasporangium : Each ovule is an oval and anatropous structure consisting of a central mass of parenchymatous tissue, the nucellus, surrounded by a two lipped protective covering the integument which is united with nucellus except at the micropylar end where it prolongs to form a short tube beyond the nucellus. A small space is left in the upper region of nucellus below the integument, which is known as pollen chamber. Integument is differentiated into 3 layers although differentiation is not so distinct as in Cycas.

 

Outer fleshy layer : Made up of thin walled cells which disappears at maturity.

Middle stony layer : Very conspicious.

Inner fleshy layer : Inner fleshy layer is well developed.

At the apex of the nucellus, a hypodermal cell gets enlarged and differentiated, it is called archesporial cell. The archesporial cell divides periclinally into an upper tapetal cell which forms tapetum, the nourishing layer, and the lower megaspore mother cell. This megaspore mother cell (sporogenous cell) divides reductionally to form a linear tetrad

Micropyle

 

Pollen chamber Pollen tube

Outer fleshy layer Nucellus

Inner fleshy layer Archegonium

Female gametophyte

Middle stony layer

 

of haploid megaspores. Out of the four megaspores, three lying towards the micropyle degenerate. The chalazal one matures into a functional megaspore.

Exointine

Exine

 

Intine

Stalk cell

 

 

Fig : Pinus : L.S. of ovule showing archegonia and pollen tubes

Body

 

  • The gametophyte : The sporogenesis results in the formation of micro and megaspores representing the first

Saccus

(A)                      (B)

Prothallial cells        Generative cells

cell

Pollen tube

 

Tube cell

 

16

(C)                      (D)

(E)

 

Fig : Pinus : (A)-(E) Various stages of microgametogenesis

 

 

 

gametophyte cells. They undergo gametogenesis so as to form the male and female gametophytes respectively.

  • Male gametophyte : The unicelled microspore undergoes three divisions of mircogametogenesis, so as to form a four-celled pollen grain or microgametophyte or male gametophyte. There are two prothallial cells, a generative cell and a tube cell. The pollen grains, at maturity are protected by three wall layers. The outermost wall layer, called exine or cappa is cutinised. the second wall layer is called exointine or capulla. It forms two balloon like outgrowths, on either side, called wings or saccus. The third wall layer is thin and called intine or tenuitas. A maturity the microsporangia dehisce by a longitudinal slit and the pollen grains are dispersed at 4 – celled stage. Since, a large number of grains are set free from a cluster of male cones in the form of pale-yellow cloud the phenomenon is often described as shower of sulphur or shower of golden dust.
  • Female gametophyte : The functional megaspore enlarges. A vacuole develops in the centre and then its nucleus divides freely to form about 2000 Initially, multinucleate tube like cells are formed called alveoli. Later, wall formation starts from periphery and proceeds towards the centre. As a result, cellular female gametophyte or female prothallus or megagametophyte or endosperms is formed. The cells of the nucellus surrounding the female gametophyte now get modified and form a nutritive layer called endosperm jacket or spongy layer. The ‘endosperm’ of Pinus is a haploid gametophytic tissue formed before fertilization.

Archegonium : Near the micropylar end, one to five archegonia are differentiated in the prothallus. Each archegonium at maturity consists of eight neck cells arranged in two tiers of four each and a venter having a small ventral canal cell and a large egg. The ventral canal cell disorganizes before fertilization. Neck canal cells are absent.

  • Pollination : The pollination in Pinus is anemophilous. The wings of pollen grains are helpful in Just before pollination the female cone axis elongates separating megasporophyll from each other. This fascilitates pollen grains to reach ovules. There is a long interval of about a year between pollination and fertilization.

 

  • Post pollination changes in the male gametophyte : The exine ruptures and the intine protrudes out to from the pollen tube that grows through the nucellar

Male gam Egg nucle

 

Fertilization Oospore

1 nuclear

2nd nuclear

 

tissue. Simultaneously, the generative cell divides to form a stalk cell and body cell. The body cell then divides to form two male gametes, which are non-flagellate.

  • Fertilization : The mode of fertilization was discovered by Goroschankin (1883). After reaching the neck of the archegonium, the tip of the pollen tube ruptures releasing the two male The ventral canal cell degenerates and the neck cell split apart. Out of the two, one male gamete fuses with the egg to form the zygote. The second male gamete along with the stalk and body nuclei degenerates.

 

 

(E)

8-celled proembryo

 

 

(G)

16-celled proembryo

Upper tier Rostate tier   

Suspensor cells

elongating

division

 

(F)

  • celled proembryo

Rosette cells

 

Primary suspensors

Secondary suspensors

division

 

 

Primary suspensor

Rosette cells

 

Embryonal tier

Fig : Pinus : Embryogeny

 

 

 

  • Embryogeny : The proembryonal development in Pinus was studied by Buchholz (1918). The zygotic nucleus moves toward the base and then divides to form four These nuclei organise into four quadrately arranged (diagonally opposite) cells with open upper end. The four cells divide simultaneousing thrice to form four tiers of four cells each. These tiers are designated from top downwards as open tier, rosette tier, suspensor tier and apical tier. Since only a part of the oospore is involved in the formation of the embryo, the development is said to be meroblastic.

As there is no cell wall on the micropylar end, the cells of open tier provide nutrition to the remain tiers of the proembryo. The cells of this tier do not divide. The cells of rosette tier divide in various planes. They simply conduct the food obtained by the cells of open tier. The cells of suspensor tier elongate pushing the embryonal cells into the ‘endosperm’. The four suspensor cells due to considerable elongation may become coiled. These cells may divide transversely to form secondary suspensor or embryonal tubes. By another transverse division, two whorls of embryonal tubes, designated as first and second series of embryonal tubes, are formed. All the four cells of embryonal tier separate from one another and develop into four independent embryos. The formation of more than one embryos from one oospore is called cleavage polyembryony. Another type of polyembryony found in Pinus is simple polyembryony i.e. when more than one embroys are developed as a result of fertilization of different archegonia. Thus in Pinus although both types of polyembryony are found but at maturity seed contains only one embryo as food is not sufficient for survival of many embryos. The embryo soon gets differentiated into radicle, plumule, hypocotyl and cotyledons. The number of cotyledons is always more than two (Schizocotyly).

  • Seed formation : Seed of Pinus is winged. The wing develops from the upper surface of ovuliferous scale. Seed has thin withered outer coat, which is pieled off, a stony coat, papery coat, cap like perisperm and food laden endosperm which encloses a central embryo. Embryo possesses 9–14 cotyledons (roxburghii). A seed represents three generations – parents sporophyte (tesla, tegmen and perisperm, if present), new sporophyte (embryo) and female gametophyte or endosperm.
  • Seed germination : The seeds may remain dormant for several years. The germination of seed occurs when the environmental conditions are The radicle protrudes out through the micropyle and enters the

Fig : Graphical representation of life cycle of  Pinus

 

 

 

soil forming the primary root. The plumule comes out and along with cotyledons it is pushed in air due to elongation of hypocotyl. The germination is, therefore, epigeal. The plumule forms a few juvenile leaves or prophylls. The juvenile leaves are spirally arranged on the branch of unlimited growth. Long shoots arise in their axis. Later on, they dry up as scales. The rate of growth of Pinus is quite slow.

(4)  Economic importance :

  • Seeds of some species are edible g., P. gerardiana (chilgoza), P. edulis.
  • Fossilized resin (amber) is obtained from succinifera and is of great commercial value.
  • Some species of Pinus are cheap source of cellulose.
  • Some species are used for manufacture of
  • Wood gas, wood tar and wood alcohol are obtained from Pinus.
  • Wood of Pinus is used for making furniture, electric poles, doors, windows, match sticks g., P. longifolia

(chir) and P. excelsa (kail)

 Cycas.                                                                                                                                                                      

Systematic position

Kingdom          –        Plantae

Sub kingdom     –        Embryophyta Phylum            –        Tracheophyta

Class               –        Gymnospermae

Order               –        Cycadales

Family             –        Cycadaceae

Genus              –        Cycas (For UP CPMT Students Only)

  • Habitat : Cycas is an evergreen palm-like It is the only genus of family Cycadaceae represented in India. Cycas has approximately 20 species found in Australia, New Zealand, Japan, China, India, Burma (Myanmar) and Pacific Islands.

In India, four Cycas species are common in Orissa, Bengal, Assam, Tamilnadu, Karnataka and Andaman.

  • Cycas revoluta : It is a native of China and Japan and is locally called Tesso. In our country, it is called ‘Sagopalm‘. Due to its primitive characters, it is also called living It is upto 10 ft tall.
  • Cycas circinalis : Plants are about 12 to 15 ft tall and distributed upto 3500 ft. In Hindi, it is called as Janglimadan mast-ka-phul.
  • Cycas rumphii : Plants are about 12 ft It is also cultivated in Indian gardens. In Tamil, it is called

Kama, Paiyindu.

  • Cycas

(2)  Structure

 

  • External structure : Fully grown plants attain a height of 2– 5 m although media attains a height of 20m. The main plant body is differentiated into root, stems and leaves.
  • Roots : Roots arise from lower part of stem and are of two types :
    • Normal roots : These form a primary tap root These roots are not green, positively geotropic with no root hairs.

 

  • Coralloid roots : From the lateral branches of the normal roots are formed dichotomously branched, apogeotropic, bluish green coralloid roots. Anabaena cycadacearum, Nostoc and bacteria are found in their cortex and sometimes enter in these roots It is an example of symbiosis. It helps in

Scale leaves

 

Megasporophylls Armour of woody

leaf bases

 

 

Foliage leaves

 

fixation and absorption of nitrogen.

  • Stem : Stem is thick, cylindrical, columnar, small, aerial and

Steam                   Bulbil

Fig : External morphology of Cycas

 

unbranched. It is covered with persistant leaf bases and scale leaves, which are found in alternate whorls. There is a crown of foliage leaves at the apex of the plant.

  • Leaves : Cycas has two types of leaves (dimorphism).
    • Scale leaves : These are reduced form of foliage leaves without lamella and are arranged in a compact spiral and alternate manner around the apex and no reproductive structures. These are protective in A single scale leaf is a brown, dry, woody, triangular structure, covered with brown hairs or ramenta.
    • Foliage leaves : These are green unpinnately compound present on the apex of the plant forming a These leaves are upto 3 metres in C. circinalis. Leaves are leathery and thick, some leaflets at the base of the rachis are reduced to spines. These are mainly photosynthetic in nature.

Leaves in Cycas show xerophytic characters.

(ii)  Internal structure

  • Root
    • Normal root : The structures of normal root resembles dicotyledonous T.S. of normal root reveals the following structures

Epiblema : This is the outermost layer with unicelled root hairs.

Cortex : Just below the epiblema is multilayered parenchymatous cortex. Some tannin cells are present in the cortex.

Endodermis : Below the cortex is present endodermis which is made up of barrel-shaped cells and below it is a layer of pericycle.

Vascular tissues : It consists of xylem and phloem which are radially arranged, i.e., on different radii.

 

 

 

Pith : It is generally absent.

Secondary growth : It is like dicotledonous plants.

  • Coralloid root : Structure of stele is similar to normal roots but cortex is divided into three zones :

Outer cortex : Having several layers of parenchymatous cells.

Middle cortex (Algal zone) : Filled with blue green algae, Anabaena and Nostoc.

Inner cortex : Having several layers of parenchymatous cells. Roots are diarch, triarch and sometimes polyarch.

 

Root hair Epiblema Parenchyma

Tanin cell

 

Endodermis Pericycle Phloem Metaxylem Protoxylem

Phellem or Cork

Phellogen

 

Outer cortex

 

Middle cortex (algal zone)

Inner cortex

Endodermis Pericycle Phloem

Metaxylem Protoxylem

 

Fig : T.S. normal root of Cycas                                                       Fig : T.S. coralloid root of Cycas

 

 

  • Stem : It resembles a dicotyledonous stem having the following tissues :

Epidermis : It is the outermost incomplete layer ruptured due to persistent leaf bases. It is made up of compactly arranged thick-walled cells.

Cortex : Cortex is large, thin-walled, parenchymatous, having a number of mucilage canals. Starch grains are found in the cortex.

Endodermis and pericycle : These layers are not very clear.

Stele : Vascular cylinder is very small having numerous small closely arranged vascular bundles, which are conjoint, collateral and open. Xylem is endarch and consists of tracheids, which have spiral thickening in protoxylem and scalariform thickenings in metaxylem. Phloem is devoid of companion cells. Albuminous cells are found in phloem.

Leaf traces : There are several leaf traces present in the cortex. Four vascular bundles enter the base of leaf, two of these are direct and other two arise from the stele of opposite side and after making semicircle, they enter the leaf. These indirect leaf traces are known as girdling leaf traces of leaf girdles.

Pith : It is large, parenchymatous and is having a number of mucilage canals. Starch grains are also found in

pith.

 

 

 

  • Secondary growth : The secondary growth in initiated by the formation of a cambium ring due to the development of interfascicular cambial strips and their subsequent joining with the intrafascicular cambia. This ring cuts secondary xylem on the inner side and secondary phloem on the outer side in additions to secondary medullary rays on both sides. This cambium ring now ceases to function another cambium now arises pericycle or inner layers of The new cambium functions in the normal way like the old one. Thus, concentric rings of secondary xylem and secondary phloem are formed. Such a wood is called as polyxylic i.e., comprising more than one xylem cylinders. Due to the presence of alternating rings of thin walled tissue (phloem) the wood of xylem remains loose and hence it is described as manoxylic. The growth in the extrastelar region takes place by the formation of a phellogen (cork cambium) which cuts off phellum (cork) on the outer side and phelloderm (secondary cortex) on the inner side. The three layers jointly constitute the periderm. The secondary growth pattern of Cycas resembles some dicots showing abnormal secondary growth. Secondary wood is devoid of vessels.

 

Armour of leaf bases

Leaf trace

 

Stele Pith

Indirect trace

Direct trace

 

 

(A)

 

 

Girdling leaf trace

 

Cuticle Epidermis Cortex

Sec. phloem 2nd ring Sec. xylem 2nd ring

Medullary ray

Sec. phloem 1st ring Sec. xylem

1st ring

Medullary ray Pith

 

Fig : Cycas, T.S. of stem (A) Outline diagram of a young stem (B) Old stem showing polyxylic condition (C) A portion of stem showing two growth rings

 

The secondary xylem is made up of tracheids showing multiseriate bordered pits. Gregus (1958) however, reported the presence of vessels in C. revoluta. Bars of sanio have been observed by Sifton, 1915 in the tracheids of

  1. revoluta. The secondary phloem comprises sieve cells and fibers. de Bary, 1884, Miller, 1919 reported the presence of sieve tubes in the secondary phloem but this needs confirmation.

(c)  Leaf

  • Rachis : In cross section of the rachis is almost circular with two depressions on upper lateral sides where the leaflets are

 

Epidermis : The outermost layer is epidermis with thick cuticle having stomata.

Hypodermis : Epidermis is followed by a well developed hypodermis, differentiated into outer chlorenchymatous and inner sclerenchymatous regions.

Ground tissue : Below the hypodermis is well developed parenchymatous ground tissue with mucilage canals. The vascular

 

Epidermis Cuticle

Chlorenchyma

 

Sclerenchyma

 

 

Leaflet

 

Vascular bundle

Mucillage canals

 

Ground

22                                                      tissue

Fig : T.S. rachis of Cycas

 

 

 

 

bundles are arranged forming in inverted omega (L). Each vascular bundle is surrounded by a sclerenchymatous sheath and is conjoint, collateral and open. In most parts of the rachis, xylem is mesarch, i.e., centripetal xylem towards periphery and two patches of centrifugal xylem one on each side of protoxylem of centripetal xylem outside the centrifugal xylem is cambium and then phloem towards periphery.

l  Leaflet

Epidermis : Epidermis is single layered with thick cuticle. The upper epidermis is complete whereas the lower epidermis is interrupted by several stomata present only in the region of blade (hypostomatic). Upper and lower epidermis are covered by layer of thick cuticle.

Hypodermis : Just below the upper epidermis, there are several layers of sclerenchymatous hypodermis while above the lower epidermis it is present only in the midrib portion.

Mesophyll : Mesophyll is differentiated into palisade parenchyma on upper side and spongy parenchyma on lower side. Palisade tissue is made up of vertically elongated cells without intercellular spaces. Both tissues contain chloroplasts.

Vascular bundle : In the midrib there is a large vascular bundle. The vascular bundle is collateral and closed. The xylem is mesarch, i.e. diploxylic condition with centripetal and centrifugal xylem.

Transfusion tissue : On each side of the midrib in between the palisade and spongy tissues is present transfusion tissue made up of horizontally arranged tracheids which supply water and mineral to mesophyll tissue

 

upto margins.

Centripetal xylem

Cuticle

Upper epidermis Hypodermis

Palisade parenchyma

 

Midrib

Fig : T.S. leaflet of Cycas (diagrammatic)

Spongy parenchyma

Centrifugal xylem

 

Phloem

Sunken stoma

 

Bundle

Hyp

sheath Transfusion tissue odermis

Lower epidermis

 

Fig : T.S. leaflet of Cycas (a portion enlarged)

 

  • Reproduction : Cycas plants are dioecious and reproduce by following methods :
    • Vegetative propagation : It occurs by means of bulbils (resting adventitious buds) which are produced on the stem in the axil of scale leaves. They break up from the parent plant and germinate to give rise to new
    • Sexual reproduction : Plant of Cycas is sporophyte (2n) and dioecious. The sexual reproduction is of oogamous type, e., takes place by the fusion of distinct male and female gametes. The male and female gametes are formed by the germination of micro and megaspores which are born on microsporophylls and megasporophylls. the microsporophylls are grouped together to form a compact conical structure called male cone, whereas the megasporophylls are not aggregated to form a cone, they are produced at the apex of the stem in succession with the leaves.

Male cone : The male cones are borne every year singly at the apex of the male plant. The growth of the male plant is, therefore, checked. Later on, a lateral bud develops which pushes the male cone to one side and

 

 

 

occupies a terminal position. The process is repeated during the formation of subsequent male cones. As such, the growth pattern of male plant is sympodial.

The male cone is a shortly stalked, oval or elliptical structure measuring about 40 – 60 cms in length. It may sometimes attain a length of 75 cms. in C. circinalis. Each cone consists of a central axis bearing numerous microsporophylls arranged in spiral manner.

  • Microsporophylls : They are wedge – shaped structures with a slightly broad base. They are soft and fleshy in the younger At maturity, they are hard and woody. They measure about 3 – 4 cms. in length and
    • – 2.3 cms. in breadth. They bear sori of sporangia on the abaxial (lower) surface. The terminal sterile portion of the sporophyll is called apophysis. In the apophyseal region the sporophyll gradually tapers and points

Cone axis Microsporophylls

Line of dehiscence

 

 

Microsporangia

 

       Bracts

  • (B)

Fig : Cycas : (A) External view of male cone

  • S. of male cone

 

 

Fig : Cycas : (a) Dorsal view of microsporophyll

  • Ventral view of microsporophyll
  • Microsporangia in sori (undehisced)

 

  • Microsporangium : The microsporangia are borne in sori on the abaxial surface of the Each sorus contains 2 – 6 microsporangia. The number of microsporangia may be upto 700 in C.circinalis, 1000 in C. revoluta and 1150 C.media. In between the sporangia are present uni or bicelled epidermal hair. The microsporangia are short-stalked, oval or elliplical structures. The development of the sporangium is of eusporangiate type. Each sporangium consists of a 5 – 6 layered wall. The outer most wall layer is called as exothecium whereas the innermost layer is the tapetum. The tapetum encloses the sporogenous tissue. The sporogenous cells divide and re-divide to form the microspore mother cells or pollen mother cells (PMC). The PMC undergo meiosis to form tetrahedral tetrads of spores. The cells of exothecium develop a thickening along their radial and inner tangential walls. The cells of tapetum and inner wall layers degenerate at maturity to provide nutrition to the developing pollen grains. The wall of a mature sporangium, thus comprises exothecium only.
  • Megasporophyll : The megasporophylls are spirally borne in acropetal order on the female plant. Since they are loosely arranged, there is no female cone Each megasporophyll is regarded as a modified foliage leaf and is

 

about 5 – 10 inches long. In the female plant therefore, the apical meristem remains unaffected. Hence, the growth pattern in the female plant is monopodial.

The megasporophylls is are flat, dorsiventral structures distinguishable into a proximal stalk or rachis part and a distal lamina. The margin of lamina is serrate or

Ovule

 

Ovule

 

  • (B) (C)

 

dentate in C. circinalis, C. beddomei and C. rumphii. In the upper part of the rachis are present 1 – 6 pairs of ovules, laterally. This number is variable in different species e.g., 1 – 6 pairs in C. revoluta, C. circinalis and only one pair in C. normanbyana.

Fig : (A)-(C) Megasporophylls of different species of Cycas

 

 

 

  • Megasporangium (Ovule) : The ovules of Cycas are largest in nature, can be seen by naked eyes. In
  1. circinalis, the ovules are largest in size, i.e., about 6 cm in length and 4 cm in diameter.

The ovules are orthotropous and unitegmic. The main body of the ovule is nucellus, covered by a single thick integument except at the top where a small opening is left called micropyle.

The integument is distinguishable into three layers, an

 

outer fleshy layer (sarcotesta), middle stony layer (sclerotesta) and inner fleshy layer (sarcotesta). The outer and inner fleshy layers are vascularised as also the nucellus by separate bundles.

Micropyle

Outer sarcotesta Middle sclerotesta

Inner sarcotesta

Integument

 

The apex of the nucellus develops a beak-like process, the nucellar beak, which projects into the micropyle.

Somewhere in the deep layers of nucellus a megaspore mother cell in differentiated. It has a prominent nucleus and dense cytoplasm. It undergoes meiosis to form a linear tetrad of megaspores. Of these, three micropyler megaspores degenerate and the lowest functions. The functional megaspore has a thick papillate outer wall called exospore and a thin, fibriller inner wall, the endospore.

Pollen chamber

 

Archegonia Nucellus

Female gametophyte or Prothallus or Endosperm

 

Vascular strands

 

Fig : L.S. megasporangium (ovule) of Cycas

 

  • The gametophyte : As a result of sporogenesis, the micro and megaspores are They are the first gametophytic cells. The microspores give rise to the male gametophyte whereas the megaspores form the female gametophyte. The gametophytes reproduce sexually.
    • Male gametophyte : The unicelled microspore undergoes two divisions of microgametogenesis and as a result three cells are formed. These three cells are serially designated as tube cell, generative cell and prothallial cell. At this stage the pollen grain is double The outer wall exine is much thicker than intine. The microsporangium dehisces by a longitudinal slit and pollen grains are dispersed at 3-celled stage.
    • Female gametophyte : The nucleus of the functional megaspore divides freely to form a free-nuclear A vacuole appears in the centre. Wall formation now begins from periphery and gradually proceeds towards the centre. As a result, cellular female prothallus or megagametophyte or endosperm is formed. The ‘endosperm’ in Cycas is a haploid gametophytic tissue formed before fertilization. This is nutritive in function. Simultaneously, a tiny space develops on the upperside of the ovule between nuclellus and the female gametophyte due to degeneration of certain nucellar cells. This is called as archegonial chamber.

Archegonium : The archegonia are formed from the gametophytic cells lining the archegonial chamber. The number of archegonia formed in a gametophyte is variable e.g., 2 – 8 in C. revoluta, 3 – 6 in C.rumphii and 3 – 8 in C. circinalis. An archegonium consists of a two celled neck but there is no neck canal cell. There is no venter either. The egg and the ventral canal nucleus remain surrounded by the cells of prothallus. Cycas produces largest egg in the plant kingdom measuring 0.5 mm. in diameter.

  • Pollination : The pollination is anemophilous. The pollen grains of Cycas are light in weight and easily blown away by wind at 3-celled stage (prothallial cell, generative cell, tube cell). At the time of pollination, a large pollination drop comes out of micropylar end of ovule by disorganisation of nucellar The pollen grains are entangled on this drop and as it dries, the pollens are drawn into the pollination chamber.

 

 

  • Post pollination changes in the male gametophyte : After a definite period of rest, the pollen grain The generative cell divides into a lower stalk cell and upper body cell. Body cell enlarges and forms several blepharoplasts, which later forms cilia.

The tube cell elongates, pierces the exine and forms a pollen tube. The pollen tube is slightly swollen and branched at tip. The pollen tube acts as haustorium absorbing food from nucellus. Body cell divides into two daughter cells and each daughter cell metamorphoses into one antherozoid or sperm or male gamete.

The male gametes of Cycas are largest (300m) in nature, visible to naked eye and are oval in form, broad (top- shaped) and naked at posterior end and spirally coiled in the anterior half with thousands of small cilia. The sperms pass into pollen tube and reach the tip of the tube.

  • Fertilization : After reaching the archegonial chamber, the tip of the pollen tube ruptures releasing the two male gametes. Besides, the tube also discharges a fluid having high concentration. When an antherozoid touches the neck cells, it is sucked in violently. By the time the ventral canal nucleus has already degenerated. As a result of syngamy, the zygote is The fertilization in Cycas is, therefore, siphonogamous (by pollen tube) accompanied by zooidogamy (by flagellate gametes). Thus the fertilization brings to an end of the gametophytic generation and the zygote is the initial stage of sporophytic generation.
  • Embryogeny : The zygote, which is the first sporophytic cell, undergoes free-nuclear divisions. A vacuole develops in the centre pushing the nuclei to the peripheral position. In the upper region there are only a few nuclei but the lower region contains numerous This is followed by wall formation that begins from periphery and proceeds to centre (centripetal). The cellular proembryo so formed soon gets differentiated into three regions –

Upper : Haustorial region, Middle : Suspensor region and Lower : Embryonal region.

Proembryo forms almost all part of embryo. Suspensor cell elongates and pushes the proembryo down into the food laden tissue of the gametophyte. Suspensor continue to elongate till they form a exceedingly long, tortuous and often spirally coiled structure. Proembryo forms, plumule and two cotyledons. Tip of suspensor forms radicle. As there are several archegonia, several developing embryos may be found in one young seed (polyembryony) but only one remains at maturity and others perish (potential true polyembryony).

  • Seed formation : The mature seed of Cycas is an orange-red or reddish-brown structure. The seed is covered by a thick testa. It is sweet in taste and emits pleasant odour. These two characteristics are responsible for the their zoochorus, (ornithochorous) Major parts of nucellus and inner sarcotesta are used up by the developing embryo reducing them to thin, papery layers.

The seed of Cycas comprises tissues of three generations namely parent sporophytic (seed coat and nucellus), gametophytic (endosperm) and second sporophytic (embryo). The embryo is distinguishable into a haustorial tip, a long suspensor, radicle, hypocotyl, plumule and two cotyledons.

  • Seed germination : There is hypogeal germination of Cycas In germination, the radicle forms a tap root. The cotyledons remain in the endosperm under the surface of soil. The plumule grows up and forms some scale leaves and later foliage leaves. Cycas seed represents 3 generations :

Old sporophytic generation (represented by seed coat and nucellus), Female gametophytic generation (represented by endosperm), and Future sporophytic generation (represented by embryo).

Life history of Cycas is diplohaplontic. It shows heteromorphic or heterologous type of alternation of generations.

 

(4)  Economic importance

Fig : Graphical representation of life cycle of Cycas

 

  • A starch called sago is obtained from the pith of Cycas, that is why Cycas is called sago palm. In Japan starch extracted from stem of revoluta is used for preparing saboodana.
  • Seed of some Cycads are used as fodder for animals.
  • Leaves are used for making mats and
  • Cycas is an ornamental
  • Boiled young leaves are eaten as
  • Extract of young Cycas leaves are used in the treatment of many skin The decoction of seeds is used as purgative. Tincture prepared from its seeds is used by Indians in headache, nausea, bad throat, etc.

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