Chapter 11 Plant Kingdom Part 2 by Teaching Care online coaching classes
- Plants generally
- Forms filamentous to
- Cells showing eukaryotic organization.
- Chief pigments – Chlorophyll a, d is present but chlorophyll c is absent; a, b–carotene, lutein, violaxanthin, fucoxanthin, myxoxanthin, g–phycoerythrin, g–phycocyanin and
- Reserve food – Floridean starch, galactan –SO4
- No zoospore
- Male gametes non-flagellate.
- Sexual reproduction by specialized type of
- Life cycle haplobiontic or
(v) Myxophyceae (Cyanophyceae)
- Plants generally fresh water, a few forms
- Forms unicelled to
- Cells showing prokaryotic
- Chief pigments – Chlorophyll a; b-carotene; luteins, myxoxanthin, oscillaxanthin, c-phycocyanin, c-phycoerythrin,
- Reserve food – Cyanophycean starch (glycogen) and cyanophycin (protein).
- No zoospore
- No flagellate
- No sexual reproduction.
- Green algae as a ancestors of land plants : There are sufficient evidences which suggest that the land plants have originated and evolved from algae. Morphological, cytological and biochemical similarities between the green algae and the land plants suggest that the ancestral algae was green, flat and fleshy with heterotrichous filamentous habit similar to Fritschiella (Smith). The various evidences which suggest algal origin of land plants are :
- Presence of similar type of photosynthetic pigments – Chlorophyll a, Chlorophyll b and carotenoids in both green algae and land
- Presence of cellulose and pectose as constituents of cell wall in both the
- Accumulation of starch as reserve food
- Similar structure of flagella in green algae and land
(11) Economic importance
- Useful aspects
- Nitrogen fixation : Some fifty species of blue-green algae are capable of fixing atmospheric nitrogen in the soil g., Anabaena, Aulosira, Cylindrospermum, Calothrix, Gleotrichia, Nostoc, Scytonema, Stigonema, Tolypothrix etc. Under aerobic conditions, nitrogen is fixed by heterocysts only. The fixation is brought about by the enzyme nitrogenase. Under anaerobic condition the vegetative cells also show nitrogenase activity.
- Algae as food : Many green algae such as Chlorella, Ulva, Caulerpa, Enteromorpha, etc. are used as food. Chlorella, a unicellular green alga, possesses a high quality of food value. It has about 50% protein and 20% of lipid and carbohydrates. The Chlorella protein contains all the amino acids essential for human nutrition. Besides, it contains vitamins A, B, C, K and various other essential elements. Ulva is collected and processed as food Ulva lactuca has formerly used in salad and soup in Scotland.
- Green algae in space research : In recent years biologists have realized that unicellular green algae (g., Chlorella) could be used to provide O2 during space flight trips. The alga can reuse CO2 during the process of photosynthesis and release O2 for the use by Astronauts.
- Antibiotics : The genus Chlorella yields an antibiotic chlorellin, which is used against Gram +ve and Gram –ve bacteria, especially Escherichia coli, Shigella dysenteriae and Staphylococcus aureus. The genus Caulerpa also yields
- Alginates : Alginic acid is a polymer of carbohydrate. It occurs in the cell wall and middle lamella. The alginates particularly ammonium, Fe, Na and K, salts are water soluble. They are obtained from Laminaria, Ascophyllum, Fucuc, Nereocystis, Turbinaria They are viscous, gel-forming and non-toxic. Hence they are used in pharmaceuticals as emulsifiers and stabilizers as well as for making pills, antibiotic capsules etc. They are also used in the preparation of soups, jellies, cosmetics, toothpastes, polishes, hair dyes, compact powders, lotions, shampoos etc.
- Carrageenin : It is a polysaccharide colloid (phycocolloid) obtained from the red algae Chondrus crispus and Gigartinia stellata. It is widely used in soups, sauces, milk shakes, cheese, jellies, cream and fruit It is also used in painting and printing.
- Kieselguhr or Diatomite : The fossil deposits of unicelled alga, diatoms are formed due to their highly siliceous cell wall (frustules). This is called as diatomite or diatomaceous earth. It is used in making sound proof buildings, lining furnaces and boilers, as insulating material and also as a
- Agar-agar : It is a non-nitrogenous carbohydrate consisting of two polysaccharides namely agarose and agaropectin. It is obtained from several red algae g., Gracilaria, Gelidium, Gigartinia, Pterocladia, Chondrus, Furcellaria, Phyllophora etc. It is insoluble in cold water but soluble in hot. It is used as a base for a variety of culture media.
- Source of minerals and elements : The members of brown algae called ‘kelps’ have been the source for obtaining iodine g., Laminaria, Macrocystis, Fucus. About 25% of total iodine is extracted from kelps. Besides iodine, the kelp also contain Bromine, Boron, Copper, Cobalt, Chromium, Iron, Manganese, Molybdenum and Zinc.
- Sewage disposal : Green unicellular algae such as Chlorella and Chlamydomonas are used in sewage disposal They remove CO2 and restore O2 by the process of photosynthesis and makes the sewage water habitable for many fishes and aerobic bacteria.
(ii) Harmful aspects
- Algal toxicity : Some dinoflagellates like Prymnesium, Gymnodinium are extremely poisonous to fishes. The blue-green alga Microcystis secretes hydroxylamine which not only kills aquatic life but also the birds and cattles who care to drink that While Lyngbya and Chlorella may cause skin allergies in human beings.
- Algal parasitism : The red alga Cephaleuros virescens causes red rust of tea thus destroying the tea Similar disease are caused by the species of Cephaleuros to coffee plant, Piper and Citrus sp.
- Fouling of marine vessels : Some brown and red algae grow on the metallic and wooden submerged parts of naval As a result, their surfaces are corroded. This creates problems in their navigation.
- Spoilage of drinking water : Forms like Anabaena, Microcystis not only spoil the taste of drinking water but also produces toxic The water filters are blocked due to growth of diatoms, Spirogyra, Oscillatoria etc. Forms like Chaetophora, Anacystis grow inside the water pipes and boilers, and thus corrode their surface by their secretion. The growth of algae is controlled by using algicides such as dichlorophen, sodium perborate, phygon XI, exalgae, delrad, cuson etc. Besides, cyanophages (LPP-1) are also used for the destruction of Lyngbya, Phormidium and Plectonema.
- Water blooms : Algae grow abundantly in water reservoirs where excess of nutrients are available to This algal growth floats on the water surface and look like foam or soap lather. It is called water bloom. e.g., Member of cyanophyceae (Microcystis, Anabaena, Oscillatoria etc.) are common. Water bloom deplete oxygen of water reservoirs and therefore, aquatic animals die of deoxygenation.
- Thallophytes : The term was coined by Endlicher (1836) for placing algae, fungi and bacteria in it. Plant body of thallophytes is called thallus. It does not show differentiation of stem, leaves and roots. An embryo stage is absent. Sex organs are nonjacketed and basically They are most primitive members of plant kingdom.
- Algae : The term was coined by Linnaeus (1754) for hepaticae and others but was used for its present meaning by L. de Jussieu (1789).
- The element present in thyroxin is obtained from Laminaria.
- In blue green algae photosynthesis process is take place in chromatophore.
- Father of Indian Phycology : O.P. Iyengar.
- Zoochlorella : Chlorella species endozoic and living as symbionts, g., C. parasitica (in Spongilla), C. conductrix (in Paramecium and Hydra).
- Red algae secrete and deposite calcium carbonate and appear like corals.
- Phycocolloid : It is mucopolysaccharide present in and over the walls of brown and red algae like alginic acid (brown algae), agar and carrageenin (red algae).
- Multicellular algae are evolutionary older than land
- Brown algae bear refracting vessels called as fucosan
- Red alga, Rhodymenia is called sheep’s
- Eye spot absent in somatic cells of
- Conducting tubes or trumphet hyphae are present in They are similar to sieve tubes.
- Red sea is due to excessive growth of blue green alga (Trichodesmium erythrium) over the surface of sea
- The algae growing attached to the bottom of a water body is called
- The first algal antibiotics chlorellin was extracted from
- Chondrus (Irish moss) is used in the prepration of various pharmaceuticals including laxatives and
- Balls of Nostoc commune are used as a food by Chinese and South Food is called Yoyucho.
- Some species of Aulosira and Anabaena are inoculated in ponds to check the development of mosquito
- Some species of Anabaena and Tolypothrix help in conservation of soil, thus checking soil
- Cymbella used as a bacterial filter.
- The common names of some important alga are given below :
Hydrodictyon (water net), Volvox (Rolling alga), Batrachospermum (Frog spawn), Ulva (Sea lettuce), Chara (Stone wor), Spirogyra (Pond scum/ water silk), Sargassum (Gulf weed).
- Introduction : Bryophyta (Gk : Bryon = moss ; phyton = plants) includes the simplest and primitive land plants. De Jussieu (1789) placed mosses etc. under acotyledons along with algae and fungi. De Candolle (1813) placed liverworts and mosses in the class Aetheogamous of the division Cellulare. Robert Broun include algae, fungi, lichen and mosses under bryophyta. It occupies a position intermediate between algae and Due to peculiar type of their habitats, they are regarded as ‘the amphibians of the plant kingdom‘.
- Habitat : Bryophytes usually grow in moist and shady places. The plants grow densely together and form green carpets or mats on damp soil, rock, walls, barks of trees and on decaying logs in forests, especially during the rainy
- Specialized habitats : Some bryophytes grow in diverse habitats such as – aquatic (g., Riccia fluitans, Ricciocarpus natans, Riella), epiphytes (e.g., Dendroceros, Radula protensa and many mosses), saprophytes (e.g., Buxbaumia aphylla, Cryptothallus mirabilis), and in dry habitats such as dry heaths (e.g., Polytrichum juniperinum), deserts (e.g., Tortula desertorum) and dry rocks (e.g., Porella platyphylla).
(4) Gametophytic plant body
- The life cycle of bryophytes consists of two distinct phases – the gametophytic phase and the sporophytic The haploid gametophyte is dominant, long lived, green and independent whereas the diploid sporophyte is short lived and dependent upon the gametophyte. The two phases come one after the other in alternating manner and both are morphologically distinct.
- The plants are small, range from few millimetres (g., Zoopsis) to 30–40 centimetres. The tallest species may reach upto 70 cm in length (e.g., Dawsonia).
- The gametophytes are either thalloid (e., not differentiated into true roots, true stem and true leaves) or
leafy shoot having stem-like central axis and leaf-like appendages.
- The roots are completely absent and they are replaced by unicellular or multicellular thread like rhizoids. In some higher forms the multicellular rhizoids form
- The vascular tissue (e., xylem and phloem) are completely absent. In few mosses (Polytricum) the xylem like hydroids, which conduct water and phloem like leptoids, which conduct the assimilates, have been reported.
- Apical growth : The apical growth in bryophytes take place by a single apical cell or a group of meristematic cells arranged in a transverse row. In Riccia, Marchantia and many jungermanniales the apical growth takes place by a transverse row of apical cell. In mosses, it occurs by single pyramidate apical cell. In Anthoceros, on the other hand, there may be a single apical cell or a transverse row of such meristematic
- Reproduction : The bryophytes reproduce vegetatively, asexually and Various methods involve in reproduction are discussed in the following account.
- Vegetative reproduction : The bryophytes reproduce vegetatively by following methods :
- Death and Decay : Most of these plants reproduce vegetatively by gradual death and decay of the older part of the plant
- Adventitious branches : Many plants like Riccia fluitans, Reboulia, Asterella, Pellia reproduce by
adventitious branches. They separate and produce new plants.
- Tubers : Several species of Riccia, Anthoceros, Sewardiella, Asterella produce tubers which give rise to new plants on the arrival of favourable conditions.
- Gemmae : Several members, reproduce vegetatively by forming multicelled gemmae. In Marchantia, Lunularia, the gemmae are produced in gemma cups. In some liverworts, 1–3 celled gemmae are prodcued on the axis or on the ‘leaves’ or on thalli. Gemmae are also produced on the thallus of Anthoceros. Several mosses also produce gemmae on the ‘leaves’ (Bryum), or axis or rhizoids or on the protonema (Funaria).
- Leafy propagules : Some liverworts also reproduce vegetatively by forming leafy
- Primary protonema : The mosses generally reproduce vegetatively by breaking of the primary New gametophores now arise from the buds differentiated on it.
- Secondary protonema : In several mossess a secondary protonema may arise from the rhizoids or primary protonema or even from the injured sporophyte. It may produce buds which give rise to new
- Rhizoids : Mosses may also reproduce vegetatively from the rhizoids g., Leucobryum.
- Sexual reproduction : The male sex organs is called as antheridium and the female as archegonium. The antheridial stalk is very distinct whereas the archegonial stalk is generally They may be of embedded type e.g., Riccia, Anthoceros or of projecting type e.g., Marchantia, mosses.
- Antheridia : They are generally borne on the dorsal surface of the While in Riccia the antheridial chambers are open, in Anthoceros they are closed. The antheridia lie embedded within the thallus in both the plants. In Jungermanniales the antheridia are borne in one or more rows. In Marchantiaceae they are present on a special branches, the antheridiophores or the male receptacles which may be stalked or sessile. While in most of the members the antheridia are superficial in origin, in Anthoceros they are endogenous. Each antheridium is distinguishable into a stalk and the body. The antheridial body consists of a mass of androgonial cells covered by a 1-cell thick sterile jacket. The terminal cell of the jacket, when distinct, is called as operculum. Each androgonial cell finally behaves as androcyte mother cell. The androcyte mother cell then forms two androcytes (antherozoid mother cell), each of which is metamorphosed into a biflagellate antherozoid.
- Archegonia : These are also borne on the dorsal surface of the In Riccia and Anthoceros they lie embedded in the thallus. In many members of Marchantiaceae they are borne on special branches called archegoniophores or the female receptacles, that may be stalked or sessile. The archegoniophore or carpocephalum has rows of archegonia protected by involucre or perichaetium. The archegonia are flask shaped structures distinguishable into a long neck and a globular, swollen venter. A multicelled stalk is also present in mosses but in others it is very short. The neck is one-cell thick. It is generally made up of six vertical rows
of cells but in Jungermamnniales it is compose of 4 or 5 vertical rows only. The neck is capped by four cover cells and contain varying number of neck canal cells inside. While in Riccia there are only 4 neck canal cells, the mosses however, possess more than six of them. The venter is also 1-cell thick in most of the plant but in Jungermanniales it is 2–3 layered. In mosses it is double layered. The venter contains an egg and a ventral canal cell.
- Importance of water in bryophytes : The bryophytes are fundamentally terrestrial plants but require presence of water to complete their life cycle. The water is needed for dehiscence of antheridia, liberation of antherozoids, transfer of antherozoids from antheridia to archegonia, opening of archegonial neck, and the movement of antherozoids into the archegonial
- Fertilization : Before fertilization the walls of androgonial cells disorganise to form a mucilagenous mass. The opercular cell is removed and the antherozoids are liberated. The neck canal cells and the ventral canal cell also The cover cells split apart giving a free passage to incoming antherozoids. The antherozoids are attracted towards the egg by chemotactic stimulus, which in bryophytes, is provided in the form of sugars. Antherozoids enter in to archegonia and fertilized the egg.
- The diploid fertilized egg (zygote) is the first cell of sporophytic It divides and develops into a sporophytic plant body, called sporogonium.
- The wall of venter forms calyptra, which provides a protective covering to the developing sporogonium.
- The sporogonium, in most of the cases, is differentiated into foot, seta and
- The sporogonium is completely dependent on the gametophyte for water and mineral supply and, in most of the cases, partly or wholly for organic The sporogonium remains attached to the gametophytic plant body throughout its life.
- The sporogonium is mainly concerned with the production of asexually formed haploid spores (or meiospores). The spores are produced inside the capsule of sporogonia as a result of meiosis in the spore mother
- The spores are the first cells of gametophytic They germinate to produce the gametophytic plant body either directly or through a juvenile filamentous stage, called protonema.
- Alternation of generation : Bryophytes exhibit a distinct and heteromorophic alternation of generations in which two phase gametophytic and sporophytic follow each other in regular sequence. The sporophytic plant body (2 N) of bryophytes is dependent on the gametophyte (N).
- Classification : Eichler (1883), Engler (1892), Bower (1935) divided Bryophyta into two classes – Hepaticae and Musci. On the other hand, Campbell (1940), Smith (1955), Takhtajan (1953) divided into three classes namely Hepaticae, Anthocerotae and Musci. Proskauer (1957) changed the names of these classes in accordance with the recommendations of the code, into Hepaticopsida, Anthocerotopsida and
(12) Salient features of classes
- Hepaticopsida : The latin word Hepatica means liver. Thus the members of hepticopsida are popularly known as liverworts. The important characters of hepaticopsida are :
- The gametophytic plant body is small, dorsiventral, thallose or leaf axis (foliose).
- Chlorophyllous cells contain many chloroplasts and one to several oil bodies.
- Pyrenoids are absent.
- Rhizoids are
- Sex organs develop from single superficial cells.
- Sporogonium has little or no chlorophyllous tissue and stomata.
- The capsule is not It lacks columella and intercalary meristem.
- Capsule dehisces by drying of capsule-wall, usually by more than two
- Anthocerotopsida : This class is characterised by the following characters –
- Gametophyte is Thalli are lobed, dorsiventral, internally homogenous without any differentiation of tissues.
- Air chambers and air pores are absent but mucilage cavities may be
- Rhizoids are only smooth
- Scales are
- Each cell possesses single (some times more) large chloroplast with central
- Oil bodies are
- Antheridia are endogenous in origin, borne singly or in groups inside the closed
- Sporogonium is differentiated into foot, meristematic zone and capsule (the seta is absent).
- Capsule has central sterile
- The capsule dehisces basipetally by two valves and shows hygroscopic
- Bryopsida : The members of bryopsida are commonly known as mosses. The class is characterised by the following characters –
- Gametophyte is differentiated into two stages – prostrate protonema and erect radial leafy
- Leaf-like appandages are spirally arranged on stem – like
- Rhizoids are multicellular with oblique septa.
- Sex organs develop from superficial cells.
- Sporogonium is differentiated into foot, seta and
- Wall of capsule is several layered with stomata on
- The capsule has central
- Elaters are
(13) Economic importance
- Soil conservation : Mosses grow in dense mats over the soil surface. They bind the soil particles and prevent soil erosion by running
- Formation of soil : Mosses along with lichens play a very important role in the formation of soil over the bare rocky They grow on rocks and add organic matter to the substratum after their death. It makes the rock surface suitable for the growth of higher plants.
- Use in nursery : The Sphagnum plants have magnificent property of retaining water. They can with hold water two hundred times more than their own Hence they are widely used by gardeners to keep cut plant parts moist during transportation and propagation.
- Peat : Sphagnum plants grow as semiaquatic or submerged in acidic marshes. The older portions of plants die but do not decay due to peculier germicidal properties. Constantly increasing mass of dead remains accumulate year after These dead remains are slowly compressed and become hardened due to weight and forms a compact dark coloured peat rich in carbon. The peat is dried, cut into pieces and used as fuel. It is mixed in clayey soil to keep it porous and mixed in sandy soil to improve its water holding capacity.
- Other uses : Certain bryophytes are used to obtain a number of antibiotic Some bryophytes grow in specialized areas and, therefore, used as indicator plants. Some bryophytes have important medicinal uses. For example – The tea prepared from Polytrichum commune is used to dissolve kidney and gall bladder stones.
- Bryophytes : The term coined by Robert Braun (1864). Nonvascular
- Bryology : The science dealing with S.R. Kashyap is father of Indian Bryology.
- The golden mine of liverworts is western Himalayas.
- Sporogonium : Sporophyte of bryophytes which is parasitic over gametophytic plant body and is mainly meant for producing
- Embryophytes : The term was used by Engler (1886) to include all plants outside thallophytes which possess an embryo stage in life cycle and jacketed sex
- Archegoniatae : The group of embryophytes having viz. bryophytes, pteridophytes and gymnosperm.
- Alternation of generation : First reported by Hofmeister (1851).
- Exoscopic : Embryo in which the apex is towards tip of
- Endoscopic : Embryo in which the apex is towards the base of
- Sphagnol: Distillate from peat tar which can treat skin
- Elaters exhibit
- Riccia fluitans, Riella and Ricciocarpus are aquatic
- Funaria is also known as cord
- A capsule of Marchantia forms about 3,000,000
- Pogonatum sporogonium is without
- Calyptra is loose cap-like portion of capsule.
- Flat roof of columella is called epiphragm.
- Central sterile column of capsule is called columella.
- Bryophytes are non-vascular plants with multicellular sex organs.
- Mature Riccia sporogonial cells are always
- Erect gametophyte of bryophyte is called
- Sterile hairs mixed with sex organs are called
- Smallest bryophyte is
- Tallest bryophytes is
- Dimorphic rhizoids are found in
- Sporophyte is completely depend on gametophyte in Funaria.
- Appendiculate scales develops in
- Common name of some true mosses are, Funaria (cord moss), Sphagnum (Peat moss/ Bog moss/ Truf moss), Polytricum (Hair cap moss), Andraea (Granite moss), Grimmia (Black moss), Tortula (Twisted moss).
- Introduction : The pteridophytes (Gk. Pteron = feather and phyton = plants ; means plants with feather like fronds or ferns). They are flowerless, seedless, spore producing vascular plant which have successfully invaded the land. Pteridophytes represent an intermediate position between bryophytes and spermatophytes (Gymnosperm and Angiosperm). It is classified by Carolus Linnaeus (1754) under the class cryptogamia. They are also called vascular cryptogams. The group has a long fossil history. It is the most primitive group that flourished in Devonian and Carboniferous periods of Palaeozoic
- Habitat : The plants of pteridophytes are mostly They prefer shady habitats. Some species of Selaginella and Adiantum are xerophytes. A fern, Acrostichum aureum is a halophyte. Some species e.g., Selaginella oregana, Psilotum flacidum, Lycopodium squarrosum and ferns like Asplenium nidus, Pleopeltis sp. are epiphytes. Marsilea occurs as a terrestrial, amphibious as well as an aquatic plant. There are true aquatics ferns like Salvinia, Azolla and Ceratopteris.
(3) Sporophytic plant body
- The main independent plant body of pteridophytes is sporophyte. It is differentiated into true roots, true stem and true
- The primary root is short lived. It is replaced by adventitious roots. The root has a permanent growing
- The stems are usually herbaceous (except in some woody ferns) and branched monopodially or
- The leaves may be small microphyllous (g., Lycopodium, Equisetum) or large macrophyllous (e.g., Pteridium, Pteris and other ferns).
- All the vegetative parts possess vascular tissues (e., xylem and phloem) organized in definite groups or steles. Secondary growth does not occur in most of the living pteridophytes (except in Isoetes).
- Apical growth : The pteridophyte generally possess a single apical cell with three cutting faces in the shoot The root tip also has a single apical cell but with four cutting faces.
- Spore producing organs : The plants of pteridophytes are They reproduce asexually by forming spores in sporangia. They are homosporous but a few plants are heterosporous also e.g., Isoetes, Selaginella, Marsilea, Regnellidium, Pilularia, Azolla and Salvinia. In Selaginella the sporangia are borne in relation to sporophylls which constitute a strobilus. In Equisetum they are borne on sporangiophores which constitute a cone. In ferns the sporangia are borne in sori on the sporophylls. The sori are of three types –
- Simple sorus : Here all the sporangia mature at the same
- Gradate sorus : Here the oldest sporangium lies in the centre and the sporangia on either side show successively younger
- Mixed sorus : It shows mixed arrangement of younger and older
In Marsilea, Azolla, Salvinia etc. the sori are present in a box like structure called sporocarp. The sorus may be naked or covered by an inducium. The inducia may be true or false. A true inducium is a specially developed structure whereas a false inducium is formed by curving of the sporophyll margin.
- Sporangium : The sporangia are generally stalked Each sporangium is distinguishable into a jacket enclosing a mass of sporogenous tissue. The sporangial jacket may be 2-4 layered. The innermost wall layer is the tapetum. No tapetum is formed in Psilotum and Tmesipteris. It is a nutritive layer which degenerates at maturity of the sporangium. The sporangial jacket in some ferns shows distinctions of annulus and stomium. On the basis of development the sporangia have been classified by Goebel, 1881 into two categories as under :
- Eusporangiate type : Such a sporangium develops from a group of superficial initials. They divide periclinally into outer and inner components. The outer cells form the wall whereas the inner cells give rise to sporogenous
- Leptosporangiate type : Such a sporangium arises from a single superficial initia It divides periclinally into outer and inner components. While the inner cell forms the stalk, the outer gives rise to sporagnium proper. In Marattia alata, the sporangia in a sorus may fuse to form a synangium.
- Spore : The plants may be homosporous, e., produce only one type of spores (e.g., Lycopodium, Pteridium) or heterosporous i.e., produce two different types of spores, smaller microspores and larger – megaspores (e.g., Selaginella, Marsilea etc.). The spore germination is homosporous pteridophytes may be bipolar (e.g., Lycopodium, Equisetum) or tripolar (e.g., Hymenophyllum) or amorphous (e.g., Angiopteris).
The spores germinate to produce haploid gametophyte, called prothallus. The homosporous pteridophytes produce bisexual (monoecious) gametophytes whereas heterosporous one produce unisexual (dioecious) gametophytes.
- Sex organs : The archegonia and antheridia are generally of embedded type. The archegonium consists of neck which usually projects from the surface of the prothallus. It contains 1-2 neck canal cells. There is no venter. The egg and the ventral canal cell remain surrounded by the cells of The antheridia are generally sessile. They have a 1-cell thick jacket enclosing a mass of androgonial cells. They form the androcytes which are metamorphosed into biflagellate (Lycopodium, Selaginella) or multiflagellate (Equisetum, ferns) antherozoids.
- Fertilization : Before fertilization the walls of androgonial cells get dissolved and antherozoids The neck canal cells and the ventral canal cell disorganise. The neck cells/cover cells separate to give a free passage to incoming antherozoids. Fertilization is affected by water medium (zooidogamous). The antherozoids are attracted towards the egg by a chemotactic stimulus provided by the degeneration of neck canal cell and ventral canal cell, in the form of malic acid.
- Embryogeny : As a result of fertilization the zygote is formed. It divides into an upper or anterior epibasal cell and a lower or posterior hypobasal cell. In Selaginella, Lycopodium the epibasal cell forms the suspensor and the hypobasal gives rise to embryo proper. By further segmentation a quadrant and then an octant is Usually the epibasal quadrant forms the stem and root and the hypobasal gives rise to foot and root. The young sporophyte of pteridophytes is dependent upon the gametophyte for food which is drawn with the help of its foot. Like bryophytes, the pteridophytes also show heteromorphic alternation of generations.
- Stelar system : The stelar theory was proposed by Van Tiegham and Douliot (1886). Some important types of steles found in pteridophytes are :
- Protostele : Solid core of xylem surrounded by phloem, pericycle and The types of protosteles are :
- Haplostele : A protostele having a central smooth core of xylem surrounded by phloem, pericycle and endodermis g., Selaginella sp., Lygodium, etc.
- Actinostele : A protostele having star-shaped xylem core with radiating ribs g., Psilotum, Lycopodium serratum, etc.
- Plectostele : A protostele having xylem broken into parallel plates alternating with parallel phloem plates
e.g., Lycopodium clavatum.
- Mixed protostele : A protostele having several xylem groups scattered and embedded in phloem g., Lycopodium cernuum.
- Siphonostele : A stele having central It is formed by medullation (or appearance of pith) in the protostele. The types of siphonosteles are :
- Ectophloic siphonostele : The central pith is surrounded by xylem, phloem, pericycle and endodermis. The phloem occurs only outside the xylem g., Osmunda.
- Amphiphloic siphonostele : The ring of xylem is surrounded on both outer and inner sides by phloem, pericycle and endodermis g., Marsilea rhizome, Adiantum pedatum rhizome.
Modification of siphonostele
- Cladosiphonic siphonostele : A siphonostele not perforated by leaf gaps g., a few species of
- Phyllosiphonic siphonostele : A siphonostele perforated by leaf gaps – g., Nephrolepis.
- Solenostele : A siphonostele perforated by leaf gaps which are scattered but not overlapping g., Ferns.
- Dictyostele : A siphonostele perforated by several overlapping leaf Each separate strand is called
meristele. e.g., Dryopteris, Pteridium, Pteris, etc.
- Polycyclic dictyostele : A dictyostele consisting of two or more concentric rings of meristeles g., Pteridium aquilinum.
- Eustele : Much dissected siphonostele having vascular strands separated apart by parenchyma g., Equisetum.
Polysetelic condition : Presence of more than one stele e.g., Selaginella kraussiana.
- Heterospory : The sporophytes reproduce asexually producing spores in sporangia. When all the spores are alike e., almost of the same size, the phenomenon is called homospory. However, in some pteridophytes, two types of spores are formed which differ significantly in their size as also in function. This phenomenon is called as heterospory. It is seen in pteridophytes like Selaginella, Isoetes, Stylites, Marsilea, Regnellidium, Pilularia, Azolla, Salvinia and Platyzoma.
It is believed that during the course of evolution, heterothallism was followed by heterospory. Some homosporous ferns e.g., Equisetum, Ceratopteris produced two types of gametophyte, thus representing incepient heterospory.