Chapter 9 Biological Classification Part 3 by Teaching Care online coaching classes

 Rhizopus/Mucor.

Systematic position

Kingdom          –         Plantae Sub kingdom           –        Thallophyta Division            –        Mycota

Sub division      –        Eumycotina Class               –        Zycomycetes

Order               –        Mucorales

Family             –        Mucoraceae

Genus              –        Rhizopus (For UP CPMT Students Only)

Mucor (For MP PMT Students Only)

• Habitat : They are cosmopolitan and saprophytic fungus, living on dead organic Rhizopus stolonifer occur very frequently on moist bread, hence commonly called black bread mold Mucor is called dung

 

mold. Both are called black mold or pin mold because of black coloured pin shead like sporangia. Besides, it appears in the form of white cottony growth on moist fresh organic matter, jams, jellies, cheese, pickles, etc.

• Structure : The vegetative body or thallus consist of well branched, aseptate and multinucleate (coenocytic) mycelium on the surface of substratum. The mycelium is almost white when young but becomes blackish during reproductive phase. The mature mycelium is distinguishable into three types of hyphae :
• Stoloniferous hyphae : These hyphae grow horizontly on the

Sporangium Sporangio-

spores Sporangiophores

 

 

Stolon

 

Rhizoidal hyphae

Columella

 

surface of substratum. They are relatively stout and less branched than other hyphae. Certain portions of the stolons called nodes, give out rhizoids and sporangiophores.

Fig : Rhizopus – Habit sketch showing stolon, rhizoidal hyphae and sporangiophores

 

• Rhizoidal hyphae : They arise in clusters from the lower side of each node and are repeatedly branched. The rhizoids penetrate the substratum and serve as anchors for the superficial These hyphae secrete enzymes like amylase and maltase into the substratum and absorb the digested food.
• Sporangiophores : They are erect, aerial, unbranched reproductive hyphae that arise in clusters from the upper side of each node. Each sporangiophores develops single terminal sporangium which is filled with spores.

In Mucor there is no such distinction. In Mucor, the hyphae develop singly. There is no holdfast or apparant node. The hyphal wall is made up of chitin-chitosan and other polysaccharides. Inner to the chitin wall is a thin layer of plasma membrane. The granular protoplasm has many nuclei, glycogen and oil droplets, mitochondria, endoplasmic reticulum and ribosomes.

• Reproduction : They reproduces by vegetative, asexual and sexual methods.
  • Vegetative reproduction : It takes place by fragmentation. If stolon breaks accidentally into small segments, each part grows into a new

 

• Asexual reproduction : It occurs by three types of non-motile mitospores, sporangiospores, chlamydospores and

• Sporangiospores : The sporangiospores are also called aplanospores. They are thin walled, non- motile, multinucleate spores formed in a sporangium. A vertically growing mycelium acts as sporangiophore. Its tip now shows accumulation of food and The tip swells up into a vesicle which gradually enlarges. Soon the protoplasm gets demarkated into an outer dense region having many nuclei and inner vacuolated region having only a few nuclei. A septum now appears separating the outer sporangium from the inner columella. The protoplasm of the sporangium now shows formation of spores by cleavage which starts from the periphery. The sporangium dehisces irregularly due to collapse of columella and the spores are dispersed. The spores germinate under favourable conditions to form the new mycelium.
• Chlamydospores : These are the perennating spores formed when the fungus starts facing dry The protoplasm of hyphae collects at certain places, rounds off accumulates a lot of food materials and develops thick wall to become chlamydospores. They tide over the unfavourable conditions and germinate to produce new mycelia as soon as they get favourable conditions.
• Oidia : In liquid, sugary and acidic medium the hyphae form small rounded cells called oidia. They multiply by budding like The budded state is called torula stage. It takes part in alcoholic fermentation. On transfer to a suitable solid medium, each oidium forms a new mycelium.
  • Sexual reproduction : Sexual reproduction takes place by conjugation between two multinucleate but single celled gametangia. The gametes are isogamous and non-motile.

The species of Rhizopus may be heterothallic (R.Stolonifer) or homothallic (R. sexualis). But mostly heterothallic in both Mucor and Rhizopus. In homothallic species sexual union in brought about between two hyphae of the same mycelium whereas in heterothallic species it occurs between two hyphae derives from different compatible strains i.e., positive (+) and negative (–).

In R. stolonifer the sexual reproduction occurs between two hyphae of opposite strains. It has been suggested by Burgeff (1924) and Mesland et al. (1974), that when two compatible strains approach each other, the following three reactions occur in members of Mucorales :

• Telemorphic reaction : The hyphae which form the progametangia are called zygophores. In this reaction club shaped zygophores are formed. The zygophore formation is induced by the hormones trisporic acids B and C.
• Zygotropic reaction : It involves the growth of zygophores

 

from +ve and –ve strains towards each other. The growth of zygophores occurs as a result of some chemotropic response.

• Thigmotropic reaction : The changes taking place as a result of fusion or contact between the two zygophores, such as gametangial

)

 

Fig : (A)-(G) Stages in the sexual reproduction upto the formation of zygospore

 

 

 

fusion and septation, are controlled by this reaction.

The two mycelial branches growing towards each other are called progametangia. Their tips become rich in food and nuclei. They enlarge and come in contact each other. A septum is laid down separating the terminal gametangium from the proximal suspensor. The gametangium has dense cytoplasm and many nuclei whereas the suspensor has vacuolated cytoplasm with fewer nuclei. Each gametangium behaves as an aplanogamete or coenogamete. The two gametangia fuse with each other. Plasmogamy is followed by pairing of nuclei of opposite strains. The unpaired nuclei degenerate. This is followed by karyogamy. The zygospore so formed develops a dark coloured thick wall and undergoes rest. It is also believed that karyogamy is delayed till the germination of zygospore.

On the arrival of favourable conditions the zygospore germinate. The outer wall ruptures and the inner protrudes out in the form of promycelium. The promycelium grows vertically upward and forms a terminal germsporangium. It is generally believed that meiosis occurs in the germ sporangium. Each diploid nucleus forms four haploid nuclei, of which three degenerate. However, according to Cutter (1942) and Laane (1974), early nuclear divisions in the promycelium are meiotic. The germsporangium shows formation of thin walled spores which are initially uninucleate. Later on, by simple mitotic divisions they become multinucleate. The germsporangium ruptures irregularly releasing the spores which germinate to form a new mycelium. Occasionally, failure of gametangial copulation results in parthenogenous development of zygospores which are called

azygospore (parthenospores).

 

Sporangium

Chlamydospor

Sporangiospores (Aplanospores)

 

Sporangiophore

Asexual

Germination

 

Germination Spore n

Zygophore (+) Zygophore (–) n

 

Progametangium (+) Progametangium (–)

 

Melosis

 

 

Germsporangium 2n

 

Sexual

Gametangium (–) Gametangium (+)

 

Plasmogamy n +

 

 

 

Promycelium 2n

 

Zygospore 2n

Karyogamy 2n

 

Fig : Graphical representation of life cycle of rhizopus and mucor sp.

(4)  Economic importance

• Spoilage of food : Exposed bread and other food particles are spoiled by Rhizopus and Mucor
• Soft rot : Rhizopus species attack sweet potato, apple and strawberry producing soft rot or leak disease. Germinating maize grains are also
• Mucormycosis : Mucor pusillus and ramosissimus may attack internal human organs including lungs alimentary canal and nervous system

 

 

 

• Fermentad foods : They are prepared from rice and soyabean with the help of Rhizopus and Mucor g

Sufru,

• Chemicals : Citric acid prepared by Mucor from molasses, fuimaric acid and cortisone by Rhizopus stolonifer, Lactic acid by stolonifer and R.nodosus and alcohol by R. oryzae and M. javanicus.
• Antibiotic : Ramysin is produced by Mucor ramannianus.
• Waste water treatment : Growth of Mucor arrhizus removes heavy metal contamination of

 Yeast.

Systematic position

Division            –        Mycota Sub division            –        Eumycotina

Class               –        Ascomycetes

Order               –        Endomycetales

Family             –        Saccharomycetaceae

Genus              –        Saccharomyces (Yeast) (For UP CPMT Students Only)

• Habitat : Yeast is a saprophytic fungus found on substratum which is rich in sugars g. sugarcane, juice, fruits (palms, grapes), milk, etc. Some species are found on animal excreta.
• Structure : Yeast was first described by Antony Von Leeuwenhock in The yeast plant consists of a single cell which is very small and either spherical or oval in

 

shape. However, under favourable conditions they grow rapidly and form false mycelium or pseudomycelium. Individual cells are colourless but the colonies may appear white, red, brown, creamy or yellow. The single cell are about 10 mm in diameter. It

is enclosed in a delicate membrane which is not made up of fungal cellulose but is a mixture of two polysaccharides known as mannan and glycogen.

The cytoplasm in granular in appearance due to the presence of droplets or granules of fat, glycogen and volutin. The

Cell wall Mitochondrion

Cytoplasmic membrane Nucleus

Nucleolus

Food particles Vacuole

Cytoplasm

 

Fig : Electron micrograph of single yeast cell

 

volutin is nothing but nucleic acid. The glycogen is the chief reserve food material and its bulk increases during alcoholic fermentation and at times it may form as much as 30% of the weight of the yeast cell.

Yeasts are facultative aerobes i.e. they are anaerobes but can also survive under aerobic conditions and respire aerobically as well. The yeast cells secrete extracellular enzyme zymase which converts complex sugars into simple soluble sugars that can easily be assimilated.

• Reproduction : Yeast reproduces by vegetative or asexual and sexual
  • Vegetative reproduction : Yeast reproduce vegetatively either by fission or by budding
• Budding : It is the common method of reproduction in budding yeasts (g., Saccharomyces) under favourable conditions (i.e., when growing in sugar solution). During this process a small bud like out growth appears at one end of the parent cell which gradually enlarges in size (unequal division of cytoplasm takes place) The

 

 

 

nucleus enlarges and divides amitotically into two daughter nuclei. One daughter nucleus goes into the bud and the other remains in the parent cell. The nuclear membrane persists throughout the nuclear division.

Daughter

 

Developing bud

buds

 

 

 

Nucleus

 

 

 

 

Vacuole                                                Mother cell

(A)                 (B)                (C)              (D)                   (E)

Fig : Budding in yeast

The vacuole almost disappears at this stage. Gradually the bud becomes almost of the same size as the parent cell. Then a constriction appears at the base of bud and a separating wall (made up of chitin) is laid down. Sometimes a bud may produce another bud over it which is still attached to parent cell forming a false mycelium or pseudomycelium.

• Fission : It is a common method of reproduction in fission yeasts (g., Schizosaccharomyces). During fission the parent cell elongates and its nucleus divides into two daughter nuclei. The two nuclei separate apart. It is followed by a transverse cytokinesis by formation of a transverse septum which develops centripetally. The two cells separate apart and behave as uninucleate vegetative thalli.

 

Vacuole

Dividing cell

 

 

Nucleus

Divided cells

 

(A)                      (B)                      (C)                      (D)

Fig : Fission in yeast

• Sexual reproduction : Sexual reproduction in yeasts takes place during unfavourable conditions, particularly when there is less amount of

The sex organs are not formed in yeasts and the sexual fusion occurs between the two haploid vegetative cells or two ascospores which behave as gametes. The two fusing gametes are haploid and may be isogamous or anisogamous. Such kind of sexual reproduction is called gametic copulation. It is the best example of hologamy i.e., the entire vegetative thallus is transformed into reproductive body. The sexual fusion leads to the formation of diploid zygote. The zygote behaves as an ascus and forms 4 – 8 haploid ascospores. These liberate and function as vegetative cells.

Guilliermond (1940) has recognised three types of life cycle in yeasts.

• Haplobiontic life cycle : This type of life cycle is common in Schizosaccharomyces octosporous, a homothallic species. It’s cells are haploid and they multiply by fission. Two haploid cells now act as gametangia and produce tiny protuberances towards one another. They fuse with each other to form a small conjugation canal or copulation tube. The nuclei of the two gametangia move into the tube and fuse to form a diploid zygotic nucleus. The zygote so formed behaves as ascus mother cel It undergoes meiosis and then mitosis to form eight haploid

 

nuclei which organise into eight ascospores. The ascus ruptures releasing the ascospores. They enlarge and behave as independent organisms.

• Diplobiontic life cycle : This type of life cycle is found in Saccharomyces ludwigii. The cells of this yeast are diploid and they multiply by Ultimately a diploid cell functions as ascus, forms four ascospores which fuse in pairs. Each zygote grows into sprout mycelium from which vegetative cells develops as buds.
• Haplodiplobiontic life cycle : This type of cycle is found in Saccharomyces cerevisiae in which haploid and diploid both types of generations are The haploid nucleus of the ascus divides in two by mitosis and then followed by meiosis resulting in four small nuclei, the two of which being of (+) strain and the remaining two of the (-) strain. Thus four haploid ascospores are formed. The ascus wall ruptures releasing the four ascospores which start budding and produce new yeast cells. These cell are of (+) and (–) strains and function as gametes. When the two cells of different strains come together they fuse to form large yeast cell. Thus the alternation of generation takes places between the haploid – diploid generations.

 

(a)

 

Sprout cells

Vegetative cells 2n

 

Vegetative cells Meiosis

Ascus n

 

 

Ascospores

 

 

Zygote (b)

Vegetative cells n

 

Ascospores

 

 

Meiosis

 

Ascus

 

 

 

2n cells (c)

 

 

 

Zygote

 

Fig : Yeast graphical representation of life cycles

• Haplobiontic, (B) Diplobiontic, (C) Haplodiplobiontic

 

In addition to above, in Schizosaccharomyces pombe, two adjoining sister cells fuse and this phenomenon is called adelphogamy. In some yeasts e.g. Debaryomyces, the mother and daughter cells fuse to form the zygote and this phenomenon is called pedogamy.

 

 

(4)  Economic importance

• Useful activities

• Baking industry : Yeast are used in manufacture of bread. Kneaded flour is mixed with yeast and allowed to Yeast convert starch into sugars and sugar into CO₂ and alcohol with help of enzyme zymase CO₂ is released when effervescence takes place due to which bread become spongy and gets swollend and is of light weight.

 

 

• Brewing industry : Brewer’s/Beer yeast is Saccharomyces cerevisiae and wine yeast is Saccharomyces ellipsoidens. They perform alcoholic fermentation.

 

C6 H12 O6

¾¾Ye¾ast  ® C

2

Zymase

H5 OH + 2CO2

 

Glu cos e                                      ethyl alcohol

• Food yeast : Yeast from brewing industry is harvested and used as food It is rich in protein and B- vitamins (Riboflavin) Special food yeasts are Torulopsis (protein), Endomyces (fat) and Cryptococus (both).

(ii)  Harmful activities

• Fermentation of fruits and fruit juices by yeast cells makes their taste
• Parasitic species of yeast like Nematospora causes diseases in tomato, cotton and
• Parasitic yeast cause diseases in human beings (g. cryptococcois, blastomycosis and torulosis).

 Albugo.

Systematic position

Kingdom          –        Plantae Sub kingdom          –        Thallophyta Division           –        Mycota Subdivision      –        Eumycotina Class               –        Oomycetes

Order              –        Peronosporales

Family             –        Albuginaceae

Genus             –        Albugo (For Rajasthan PMT Students Only)

• Habitat : Albugo is an obligate parasite and grows in the intercellular spaces of host tissues. It is parasitic mainly on the members of families Cruciferae, Compositae, Amarantaceae and Convolvulaceae, The disease caused by this fungus is known as white rust or white blisters which is prevalent all over the The fungus forms shiny white patches on the leaves, mostly on their lower surfaces, stems and petioles. The fungus also causes hypertrophy and deformation in affected parts.

The most common and well known species is Albugo candida which attacks the members of the mustard family (Cruciferae). It is commonly found on Capsella bursa – pastoris (Shepherd’s purse) and occasionally on radish mustard, cabbage, cauliflower, etc. The reserve food is oil and glycogen.

• Structure : The plant body of the fungus is mycelial and eucarpic. The mycelium is intercellular, branched, aseptate and multinucleate (coenocytic). The mycelium produces finger like or globular haustoria which enter into the host cells to absorb the food The mycelial wall is made up of cellulose-glucan. The cells show characteristic fungal eukaryotic organization. The parietal layer of cytoplasm also contains oil globules.

 

Glycogen bodies

 

 

Mycelium

 

torium

Fungal cytoplasm Fungal cell wall

 

 

Host cell vacuole

 

Lomasomes

 

Lamasomes Fungal plasmamembrane

Fungal cytoplasm

Sheath Fungal

 

Oil drops

(A)

Host cell vacuole

Host cytoplasm

cell wall

Host plasma membrane

 

Fig : Albugo – (A) Intercellular mycelium with knob like haustoria, (B) Ultra structure of haustorium (diagrammatic)

• Reproduction : The fungus Albugo reproduces asexually as well as
  • Asexual reproduction : Asexual reproduction take place by the formation of sporangia or conidia. The fungal mycelium collects below the host epidermis and forms sporangiophores (conidiophores). These are club- shaped and multinucleate. The sporangiophores cut sporangia in basipetal manner, that is, the oldest and first formed are at the top and the youngest and last formed are at the base. The sporangia remain attached in the form of chains .The walls between the sporangia fuse to form a gelatinous disc-like structure called disjunctor. Due to formation of numerous sporangia, the epidermis of the host is raised in the form of a postule.

Finally the disjunctors are dissolved in water and sporangia are set free. They appear as white powdery mass on the host tissue. These smooth, rounded structures are disseminated by wind. Germination of sporangia occurs when they reach a suitable host. The mode of germination depends on the availability of water or even moist air.

• If water is available sporangia forms About 4 – 12 (usually eight) zoospores within two minutes of their The zoospores are kidney shaped and laterally biflagellate (unequal). The zoospores move into a vesicle before they are released. After a swimming period, the zoospores come to rest, encyst and germinate by germ tubes. The germ tubes enter into the host and produce an intercellular mycelium.
• If water is not available, the sporangia germinate directly by forming germ
  • Sexual reproduction : The sexual reproduction is oogamous type and takes place with the help of antheridia and oogonia. The antheridia and oogonia are multinucleate in the beginning but become uninucleate by disorganization of nucleri. the antheridia are club- shaped and the oogonia are globular. Both sex organs develop terminally on the hyphae. There is a single egg (oosphere) surrounded by periplasm. At the time of fertilization, a receptive papilla develops on one side of the oogonium through which the fertilization tube enters into the oogonium. Inside the oogonium the male nucleus fuses with the egg nucleus. The diploid zygote develops a warty wall and becomes the oospore. The diploid nucleus undergoes meiosis, followed by several mitotic divisions. After a period of rest, the oospore germinates and produces reniform, biflagellate zoospores. The zoopores are first released into a vesicle and then to the outside. They swim for some time, encyst and then germinate to form germ Most part of the life cycle of Albugo is gametophytic. The sporophytic phase is limited only to the oospore stage.

According to Sansome and Sansome (1974), the species A. candida is heterothallic and meiosis occurs inside the gametangia (antheridia and oogonia) and not in the oospore. Thus the vegetative mycelium of Albugo is diploid.

 

 

 

Fig : Graphical representation ofl ife cycle of rhizopus and mucor sp.

 Lichen.                                                                                                                                                                      A lichen is structurally organised entity consisting of the permanent association of a fungus and an alga. The fungal component of a lichen is called mycobiont (mostly ascomycetes) and the algal component is called

phycobiont (mostly blue green alga). Both mycobiont and phycobiont are associated in symbiotic union in which

the fungus is more predominant and alga is subordinate partner. The fungus provides the structural covering that protects alga from unfavourable conditions, i.e., drought, heat etc. It also traps moisture from the atmosphere and anchors the lichens to a rock, tree bark, leaves and other similar supports. The alga prepares organic food (e.g., mannitol) by the process of photosynthesis from carbon dioxide. If the algal component is a cyanobacterium (blue green alga), it fixes atmospheric nitrogen in addition to preparation of food. The relationship between the two is that of consortium, symbiosis or mutualism. Ahmadjian (1963) considers fungus to be a controlled parasite. The phenomenon is called helotism.

• Habitat : Lichens are cosmopolitan in distribution.Their growth is very slow. Some lichens growing in arctic regions are believed to be 4500 year old. The lichens which grow on stones are called saxcoles (e.g., Dermatocarpon) and those growing on barks are called corticoles (g., Usnea). A few liches are aquatic (e.g., Peltigera, Verrucaria margacea). The lichens generally do not grow near smoky industrial areas where atmosphere is polluted. Cladonia rangiferina, commonly known as reindeer-moss grows luxuriantly in tundra region and form the food of animals like the reindeer and caribou (musk ox). Some of the common Indian genera are: Cladonia, Parmelia, Usnea, Physcia, Anaptychia, Lecidia, etc. Lichens are highly pigmented. They may be bluish, green, grey, yellow, orange, red and brown in colour. Some are white (Gyrophora).

 

 

 

• Classification : Hole (1967) divided lichens into 3 classes :
  • Ascolichen : When fungal partner belongs to Most lichens are ascolichens. Ascolichens are further divided into :
• Gymnocarpeae : Fruiting body is apothecium
• Pyrenocarpeae : Fruiting body is
  • Basidiolichen : When the fungal partner belongs to
  • Lichen Imperfecti : When the fungal component belongs to fungi

(3)  Structure

• External structure : The lichens vary in their size and However, sthree main types are recognised on the basis of their habit, growth, form and mode of occurrence.

• Crustose or Crustaceous lichens : These lichens occur as crust over rocks, soil or tree barks, g., Graphis, Haematomma.
• Foliose or Foliaceous lichens (Leafy lichens) : They are leaf like lobed structure which attached to substratum by rhizoid like organs, g., Parmelia,, Paltigera.
• Fructicose or Filamentous lichens : They are branched shrubby lichens but small base g., Cladonia, Usnea.
  • Internal structure : The bulk portion of lichen thallus is formed by fungal The alga constitutes about 5% of the lichen body. Internally the lichens are of two types
• Homoiomerous Thalli : Algal cells and fungal hyphae are uniformly dispersed throughout the thallus,

e.g., Collema.

• Heteromerous thalli : The algal cells are restricted to algal zone In these forms fungal component is dominant. Usually the heteromerous thalli show 4 distinct zones.

Upper cortex : Formed by compactly interwoven hyphae either without interspaces between them or interspaces filled with gelatinous substances. A cuticle like layer is present on the surface. In some species e.g., Parmelia breathing pores are present.

Algal layer : Present just below the upper cortex forming photosynthetic zone of the thallus. This layer is also called gonidial layer.

Medulla : Occurs nearly in the middle of the thallus beneath the algal layer the hyphae are loosely

 

interwoven in this layer.

Upper cortex

Algal zone (gonidial layer)

 

Medulla

 

Lower cortex

 

Rhizine

 

Fig : Transverse section of a foliose lichen

Lower cortex : Comprising of closely packed dark coloured hyphae Rhizoids arise from this layer.

 

 

• Special structures and propagules : The following specialized structures and propagules are associated with lichen thalli :
  • Breathing pores : The upper surface of some lichens, particularly the foliose lichens is provided with Here the fungal hyphae are loosely arranged. They help in aeration.
  • Cyphellae : These are small, almost circular depressions present on the lower side of the thallus. The medullary hypae are not exposed through these depressions due to the presence of corticating They are meant for exchange of gases e.g., Sticta. Similar structures without any cortical border are called pseudocyphellae.
  • Cephalodia : These are gall like outgrowths present on the upper surface of the thallus. They are distinguishable into cortex and medulla with similar fungal but different algal components from that of the main Lichens having two algal and one fungal partner are called diphycophilous. The cephaloida are meant for retaining moisture e.g, Peltigera.
  • Isidia : These are coral like, simple or branched outgrowths present on the upper surface of the They have the same algae and fungal component as that of the main thallus. They help in photosynthesis as also in vegetative propogation e.g., Parmelia, Peltigera.
  • Soredia : It is a powdery mass comprising both algal and fungal components formed in a postule like structure called soralium. The soralia arise from the algal zone lying just below the upper cortex g., Physia, Parmelia.
• Reproduction : Lichens reproduce both by asexual and sexual

(i)  Asexual reproduction

• Fragmentation : The main thallus breaks into small pieces which grow as independent
• Rejuvination : Plants like Cladonia show this unique phenomenon. It becomes young again. The older parts of the thallus die whereas the young branches continue to
• Isidia : These are small superficial outgrowths on the surface of lichen thallus. They enclose the same alga as present in the thallus and covered by continuous cortex. Their function is to increase the photosynthesis by increasing the surface They get detached from the thallus, disseminate by wind and grow into new thalli.
• Soredia : This is a powdery mass formed in a postule like structure called Each soredium forms a new thallus under favourable conditions.
• Conidia : In serveral lichens the fungal component forms conidia on conidiophores. They form new fungal mycelium which with suitable algal component form the
• Pycniospores : The conidia formed in a flask shaped structures lying embedded in the thallus (pycnidia) are called as The pycniospores form new fungal mycelium which consitute the lichen on coming in contact with suitable algal component.
  • Sexual reproduction : Sexual reproduction in lichen is performed only or mainly by its fungal component. So, the structure of the reproductive organs is dependent upon the type of their fungal

 

 

 

The ascolichens reproduce sexually by forming sex organs. The female sex organ is called ascogonium and the male, pycnidium. The ascogonium is a multicelled structure coiled in its basal region. The terminal region is some what eract and called trichogyne. The ascogonium remains embedded in the thallus. The pycnidia acting as male sex organs are called spermogonia. They are pitcher shaped structures that lie embedded in the thallus. The conidia formed in the spermogonia act as spermatia. Some sterile hyphae also emerge out of the ostiole. The spermatia are colourless tiny structures of varying shapes, they are disseminated by wind. Finally, they attach themselves to the sticky tip of the trichogyne. This is followed by plasmogamy and migration of the male nucleus to the female structure. Ascogenous hyphae now develop from the fertilized cell of ascogonium. The asci develop by crozier formation and karyogamy occurs inside the ascus mother cell. This is followed by meiosis and mitosis resulting in the formation of eight ascospores inside the ascus. Simultaneously, the surrounding hyphae also develop and as a result fruiting body called ascocarp or ascomata is formed. The ascocarp may be an apothecium or perithecium.

Accordingly, the ascolichens are divided into two groups namely gymnocarpae and pyrenocarpae respectively. While the apothecia are cup- shaped structures e.g., Physcia, the perithecia are pitcher shaped e.g., Acrocordia. The fruiting body is internally distinguishable into three zones.

 

• Thecium : It is the fertile zone comprising fertile asci and sterile
• Epithecium : It is the zone formed by the tips of paraphyses projecting beyond the
• Hypothecium : It is the zone formed by loosely packed hyphae lying below the

An apothecium has two types of margins, proper and thalline. The proper margin is formed by fungal hyphae only

 

 

(B)

Asci          Paraphyses

 

Ascospore

 

 

(A)

Epithecium Thecium

Hypothecium

 

Algal layer Upper cortex

Medulla

Lower cortex

 

whereas the thalline margin includes the algal component also.

Accordingly, we differentiate two types of apothecia.

Fig : V.S. of lichen thallus through apothecia,

• Lecanorine type, (B) Lecideine type

 

• Lecideine type : They have only the proper margin g., Lecidea.
• Lecanorine type :They have both proper as well as thalline margin g., Lecanora.

The sterile tissue lying in between the asci is sometimes called hamathecium. The asci dehisce releasing the ascospores. The ascospores germinate to form the fungal hyphae. On coming in contact with the suitable algal component, they constitute the lichen.

(6)  Economic importance

• Pioneer of vegetation : Lichens are considered as pioneers in initiating a plant succession on rocks. These are the first plant group which play an important role in the formation of the So lichens are called as formers of nature or soil builders. Crustose being the first followed by foliose and finally fructicose lichens.
• Food and Fodder : Reindeer moss (Cladonia rangiferina) of the arctic region is the used as food for reindeer and Iceland moss (Cetraria icelandica) is ground up and mixed with wheat and made into cakes in Iceland. Rock tripe (Umbillicaria) has been eaten by travellers when they face starvation in actic regions. Evernia is used by Egyptians for making bread and Umbilicaria esculenta is regarded as a delicacy in Japan. Species of Parmelia are used as curry powder in India.

 

 

 

• Medicinal uses : Dog lichen (Peltigera canina) was used as medicine for hydrophobia in ancient days and Lungwort (Lobaria pulmonoria) was used for the diseases of lungs. Usnic acid obtained from Usnea (old man’s beard) and Cladonia is used as broad spectrum antibiotic. It is effective against gram positive bacteria. Lobaria pulmonaria, Cetraria icelandica are used in respirotory diseases particularly T.B., Roccella montagnei in angina, Peltigera canina in hydrophobia, Parmelia sexatilis in eipilepsy and Usnea barbata in urinary troubles.
• In perfumery : Ramalina and Evernia, having sweet scented thalli, are used in the preparation of Dhup, Havan Samagri and soap. Perfumes are extracted from Evernia prunastri and Lobaria pulmonaria.
• In tanning and dying : Lichens like Cetraria icelandica and Labaria pulmonaria are used in tanning. A red dye is obtained from Ochrolechia whereas Parmelia sp. yield a brown dye. Litmus used as acid-base indicator is obtained from Roccella montagnei and Lasallia pustulata. An orchill dye is obtained from Roccella and Leconara which is purified as orcein and used as a biological stain.
• In brewing and distilling : The lichens contain carbohydrates in the form of lichenin. Cetraria islandica and Cladonia rangiferina (yield upto 66% of the polysaccharides) are used to obtain alcohol in Sweden and

 

• Indicators of air pollution : Lichens are very sensitive to

SO2

and grow only in

SO2 free atmosphere.

 

So lichens like Usnea are used as indicators of air SO2

pollution.

 

• As poison : Some lichens are poisonous also such as Letharia vulpina due to vulpinic acid, Cetraria juniperina due to pinastrinic acid, Parmelia molliuscula due to selenium, Xanthoria parietina due to beryllium and Everina furfuracea due to chlorine.
• Other uses : Some lichen yield important chemicals g., salazinic acid (Ramalina siliquosa), Lecanoric acid (Parmelia subrudecta) and squamatic acid (Cladonia crispata) etc. In hot season, Usnea gets dry and becomes highly inflamable. It easily catches fire and causes forest fires.

 Mycorrhizae.

The term ‘mycorrhizae’ was coined by Frank (1885) It is an association between a fungus and the root of a higher plant e.g. Pine, Birch, Eucalyptus, Ficus etc. The actively growing roots of higher plants get infected by fungi. As a result, the roots are modified (i.e., become tuberous, nodulated, coralloid, etc.) to accommodate fungi. The root cells and fungi directly transmit nutrient substances to each other. Mycorrhiza is a example of symbiosis or mutualism.

• Types of mycorrhizae : Mycorrhizae are classified into three categories :
  • Ectotrophic mycorrhiza : It occurs only in about 3% of plant species, majority of which are forest trees, pines, sprues, firs, oaks, beeches, birches, eucalyptus etc. The fungus partner is commonly a basidomycetes. In this type of mycorrhizae, the fungus completely encloses the rootlet in a sheath or mantle of tissue formed of compact hyphal cells and penetrates only between the cells of root cortex. The ectomycorrhizal fungus cannot exist saprotrophically in nature without a plant host association. Such roots are devoid of root hair, root cap and may become unforked, bifurcate, nodular or coralloid.
  • Endotrophic mycorrihiza : In this kind of mycorrhizae the fungus does not form an external mantle but lines within the The ectomycorrhizae are further divided into three groups :

 

 

• Ericaceous mycorrhizae : The fungus forms dense intracellular coils in the outer cortical
• Orchidaceous mycorrhizae : These are associated with orchid roots. The fungus forms association from the time when the orchid seeds
• Vesicular-arbuscular mycorrhizae (VAM) : The fungi of this group mostly belong to zygomycetes. This type is significant in agriculture because it occurs in a large number of crop plants. The fungal hyphae develop some special organs, called vesicles and arbuscules, within the root cortical cells.

 

 

 

Fig : T.S. of a vesicular-asbuscular mycorrihza (A Asbuscules, Ap. appresorium, Ph. permanent hyphae, SLH. short-lined hyphas V. Vesicle)

 

• Ectoendotrophic mycorrhiza : This type of mycorrhiza sharing characteristics of both ecto and endotrophic The fungus forms a hyphal mantle and Hartig net as do the ectotrophic mycorrhiza and also establish haustoria and hyphae coils in the epidermal and cortical cells, like the ectorophic mycorrhizas. The external hyphae deliver organic compounds absorbed from the humus to the root cells. One of the best studied examples of ectoendotrophic mycorrhizas is the mycorrhiza of Monotrapa indica, the Indian pipe.

(2)  Advantages of mycorrhizal association

• Since all fungi are dependent on some kind of foreign organic matter for their survival, the mycorrhizal fungi obtain their nutrient requirement (primarily simple carbohydrates) from the host plant without damaging the function of root tissues,
• The fungal hyphae increase a plant’s uptake of certain nutrients from the soil, particularly phosphorus, copper, zinc, nitrogen and
• The mycorrhizal hyphae permeate the soil and help the absorption of water by host more efficiently,
• The mycorrhizal plants need less fertilizer and can even grow better on the infertile They withstand high doses of heavy metals and acid rain pollution.
• The fungi produce various growth promoting substances which help the plants to grow
• Due to mycorrhizal association, the higher plants develop resistance to soil borne diseases (due to phytolaxins released by fungi), drought resistance and tolerate salinity, pH and temperature