Describe Fungi. Discuss structure of thallus, nutrition and reproduction in them

Fungi are thallophytes of chlorophyll less non vascular plants that live parasitically or saprophytically and reproduce by spores. Branch of botany which deals with fungi is called Mycology. Study of disease caused by fungi are called plant pathology. Fungi are found every where. They have no chlorophyll and may be saprophytes, parasites or symbionts. Some fungi are aquatic. Some are terrestrial. They occur in soil. Some live in tissues of plants and animals. Fungi can grow on our food stuffs like bread, jams, pickles and fruits. They are also present all the time in air.

Vegetative structure of Fungi:
Fungi consists of network of branched filaments called hyphae. Entangled mass of hyphae is myceliym. Hyphae may be segmented or non segmented. Some fungi like yeasts (Saccharomyces) and Syachtrium lack mycelium.

Cell wall of mycelium is made up of chitin or fungal cellulose along other substances.
Chief food reserves are glycogen and oils. Fungal cells contain cytoplasm, nuclei and vacuoles. Mycelium of lower fungi is aseptate and coenocytic where as of higher fungi it is septate and cell may be uni, bi or multi nucleate.

Mycelium may be of following kinds.
(1) Piectenchyma:- When the fungal mycelium is compactly woven to form a pod like tissue called as plectenchyma. It is of two types prasenchyma in which hyphac of the mass retain their individuality and do not fuse and pseudoparachma in which hyphare are completely fused to each other and lost their individuality and the whole mass looks like the parenchyma of higher plants.
(2) Sclerotia:- In this case the interwoven hyphac of mycelium become compact that the mass becomes rounded and cushion like and is know as sclerotia.
 (3) Rhyzomorph:- These are complex structures in which hyphac grow side by side. They are dark brown gelatinous and thick. They sometime resemble the finer roots of a tree.
Nutrition:- There is lack of chlorophyll in fungi hence they are unable to synthesize food. They are heterotrophic in their mode of nutrition and they get food from external source. Thus they live either as parasites or saprophytes. Fungi that get their food from dead organic material are called sabrophytes and the fungi which obtain their prepared food from living plants or animals are called parasites. The living beings on which the fungi parasitize are called hosts. Some fungi grow in association of other plants and are mutually benefited and this process is called symbiosis.
Reproduction:- Reproduction in fungi takes place by vegetative reproduction, asexual method and sexual method.

(1) Vegetative Reproduction:- It takes place by
(i) Fragmentation:- Hyphac break up into two or more pieces due to some external force and each of them develops into new one.
(ii) Fission:- Some unicellular fungi like yeast and bacteria reproduce by fission. The vegetative cell elongates and divides into two daughter cells by constriction or transverse wall.
(iii) Budding:- The parent cell buds off new cell which separates from parent cell and develop into new individual. Eg. Yeast cells.
(iv) Sclerotia:- This is modification of mycelium. It is hard, resting storage object. It also serves as a means of perenvation and vegetative propagation on return of favourable conditions it germinates.

(2) Asexual Reproduction:-
It takes place by several types of spores as under:
(i) Motile Zoospore:- Motile zoospores may be uni or biflagellate pear shaped sporangium eg. Albugo and syachyterium.
(ii) Conidia:- These are non motile spores produced exogenously by constriction at the ends special hyphal branches called conidiophores. They may be produced singly (eg. Phythinum, phytophthora) or in chains (eg. Aspergillus & Penicillium).
(iii) Oidia:- Some fungi like mucor the hyphae forming mycelium become divided by transverse walls into large number of short segments each of which is capable of germinating and developing into a new plant. These segments which remain united in chains become free from each other and are known as oidia.
(iv) Chlamyolospores:- They are produced like that of oidia but they differ from the latter being thick walled and coloured black or brown. They may be terminal or produced at irregular intervals along the hyphae.
(v) Spores:- They are simple reproductive units usual unicellular and with certain food reserve. In higher fungi like ascomycetes and basidiomycetes ten spores-asporpores and basidiospore, are produced.

(3) Sexual Reproduction:- In fungi like other organisms sexual reproduction takes place by the union of two compatible nuclei and consists of three phases.
(1) Plasmogamy:- It involves the fusion of two protoplasts bringing two haploid nuclei class together in the same cell.
(2) karyogamy:- It involves the fusion of two haploid nuclei brought together during plasmogamy. This results in the formation of a diploid structure the zygote.
(3) Meiosis:- It follows nuclear fusion which takes place in all sexually reproducing fungi. It again reduces the number of chromosomes from diploid zygote nucleus to original haploid number in the daughter nuclei.
In fungi sexual reproduction is of following types:-
(a) Planoganetic Copulation:- Planoganetes are motile gametes. This involved the fusion of two naked gametes. The fusing gametes may be isogametes, anisogametes or heterogametes.
(b) Gametangial Contact:- In this case the gametes are transferred directly from one gametangium to the other. These gametes are undifferentiated protoplasts and are represented by nucleus. They never fuse.
(c) Ganetangial Copultion:- it involves the fusion of entire contents of two gametangia.
(d) Spermatization:- In some cases like ascomycetes and basidionycetes there is the formation of certain non motile male cells the spermatia. They are brought in contact with female. Gametangia, receptive hyphae or somatic hyphae to which they are attached. Partition wall dissolves and contents of spermatia pass into females organ.
(e) Somatogamy:- In most of the member of ascomycetes and basidiomycetes there are no sexual organs and the sexual act is a accompanied by fusion of two nuclei from somatic cells. This fusion results the formation of zygote, zygospore or oospore. They produce spores which in their turn develop into new mycellium.

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