Thallus organization of Fungi
Organization of Thallus in Fungi
The thallus of fungi may be unicellular on filamentous:
In a number of the lower fungi like the chytrids, the thallus is more or less a spherical, single-celled structure . At the time of reproduction, it becomes a reproductive unit. The latter produces the asexual or sexual cells. Such fungi are called holocarpic. In them, the vegetative and reproductive stages do not occur together in the same thallus. Plasmodiophora features a vegetative phase consisting of a unadorned , multi-nucleate, amoeboid mass of protoplasm. It is termed plasmodium.
The protoplast of the diploid plasmodium cleaves to make the resting spores. The yeasts, which are associated with the filamentous forms, even have a unicellular thallus. In the unicellular holocarpic forms (Synchytrium), the mycelium is absent.
Filamentous Thallus-The overwhelming majority of the fungi have a filamentous thallus. It originates through the germination of a spore. The spore germinates because it lands on an appropriate substratum where other conditions of life also are favourable. In some species, the spore, on germination, produces only a brief , tubular structure of limited growth. It constitutes the thallus and is technically called a hypha. The spores of most of the fungi, however, produce to a fluffy thallus consisting of a cottony mass of fine, branched filaments. These long, fine filaments are called the hyphae (sing, hypha). a number of these hyphae, at a particular stage of maturity, extend into the air and bear the reproductive bodies. the remainder cover or within the substratum and continue the traditional activities. Such fungi are called eucarpic. Collectively the hyphae comprise the vegetative body (thallus) of a fungus which is named the mycellium. The hypha is thus a structural unit of the mycelium. It consists of a skinny , transparent wall filled or lined with a layer of cytoplasm. The medium upon which the mycelium grows is understood as substratum. The mycelium is that the food procuring structure within the life cycle. It carries on the overall activities of a plant cell like absorption, digestion, respiration, excretion and growth but not photosynthesis. The hyphae constituting the mycelium branch, spread altogether directions within or over the substratum to make a loose and ramifying network. The hyphae are usually colourless particularly those embedded within the substratum. The aerial hyphae in some fungi become coloured. Black, orange, yellow, red, blue and brown are the standard tints. the color is typically confined to the hyphal wall. Even when the pigments are present within the protoplasmic contents, they are doing not form an integral a part of the living matter. The pigments play no role within the physiology of the fungus. the expansion long takes place at the ideas of the hyphae and is thus termed apical.
Kinds of Mycelia
The fungus mycelium, in fact, is a multinucleate structure lacking complete internal cell bounderies.
In the algal fungi (Phycomycetes) the mycelium in the vegetative phase usually lacks internal partitions of any kind. The hyphae are thus multinucleate and aseptate. The mycelium is a continuous mass. It grows terminally by the apical elongation of the hyphae accompained by increase in the number of nuclei buy nuclear divisions. The aseptate, multinucleate mycelium is called coenocytic. The septa in the aseptate, coenocytic mycelium, however, are formed only to cut off reproductive structures, or to seal off a damaged portions. Such septa are solid plates. They lack pores.
The members of other classes of fungi (Ascomycetes and Basidiomycetes) develop internal cross walls called the septa which divide the hyphae into segments. The septa appear at regular intervals behind the hyphal tip. The segments may be uninucleate or multinucleate. In a septate mycelium the septa between the cells are transverse. Longitudinal or oblique septa are rare. The formation of septum is always preceded by the division of nucleus. If the cell has more than one nuclei there is simultaneous division of all the nuclei before wall formation. It must, however, be borne in mind that the septa in the septate mycelium of fungi are incomplete. Each has a central pore and rarely more than one pore. Through these the cytoplasm, cytoplasmic organelles and even nuclei can regularly pass from one cell to other. It is evident, therefore, that the distinction between coenocytes and septate hyphae is not so profound as it was formerly thought to be. The presence of septa gives mechanical support to the hyphae. Complete partitions do not occur in the vegetative phase of fungi. In some cases, the septa possess more than one pore and rarely none at all.
Origin of Septa
The septa in all the fungi (septate Phycomycetes, Ascomycetes, Basidiomycetes and Imperfect Fungi) have a similar origin.
They arise at regular intervals behind the hyphal tip. The septum originates at the periphery on the inside of tubular hyphal wall as a ring (annular) or rim of wall material. The annular growth grows slowly inwards towards to centre (centripetally) like an iris diaphragm increasing in width and decreasing the diameter of pore. A cross wall or septum is finally formed which is seldom complete. The centripetal growth of the septum in fungi is rapid. According to Buller (1933), the aseptate Rhizopus takes 20-25 minutes to completely seal off a damaged portion. The complex dolipore septum in Rhizoctonia (Basidiomycetes) is completed in 10 minutes. Ciboria takes only 6 minutes.
Usually a small hole or a pore remains in the centre of the septum. The perforated septum gives rigidity to the tubular hypha maintaining, at the same time, the protoplasmic continuity from cell to cell. In all the septate fungi with the exception of the Basidiomycetes, the septum has a simple central pore in the middle of the cross wall. The septum in some may be slightly swollen where the pore is formed. In the Basidiomycetes, there is further elaboration of the septum to produce a more complex pore named a dolipore (Latin dolium meaning a large jar). The rim of the central pore, in this case, is swollen and thickened to form a barrel-shaped structure with open ends which are guarded by cap-like covers. The latter in section appears as a round bracket or parenthesis. The septal pore cap is, therefore, called the parenthesome. The complete septal pore ordinarily permits the movement of cytoplasm, mitochondria and nuclei from cell to cell but under certain conditions the parenthesome may close the passage. The dolipore parenthesome is the hallmark of the Basidiomycetes.
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