Thallus organization of Fungi

 1.3 Thallus organization

         The vegetative structure of the fungal body cannot differentiate into root, stem, and leaf. Such a vegetative structure is called a thallus. Their body is composed of hyphae. But the structural differences are seen between lower and higher groups of fungi.

         In the primitive group, the slime molds, the thallus is a naked amoeboid mass of protoplasm which may be a plasmodium or a pseudoplasmodium. In the members of the higher fungi called eumycota, the thallus is well-developed with reproductive structures.

        Some fungi are unicellular which behaves as both vegetative and reproductive cells. For example Yeast, Blastocladiella sp., Candida  sp., etc. But in other forms, the thallus contains a network of much-branched thin filaments called hyphae. The twisted mass of hyphae is known as mycelium. In lower groups fungi such as Mastigimycotina and Zygomycotina, the hyphae are aseptate (no wall/septum) and in the case of higher groups setae are found. When the hyphae are septate (having wall or septum), there is a pore in the septum through which cytoplasmic communication between cells is established. Basidiomycetes have dolipore septum.

         The aseptate thallus or hyphae contain many nuclei in the cytoplasm and this is called coenocytic hyphae.

        `The thallus may be grouped as holocarpic or eucarpic depending on their involvement in the production of reproductive structure. When the thallus as a whole is vegetative as well as reproductive in function, it is called holocarpic. However, when a portion of the hyphae or the thallus takes part in reproduction the thallus is eucaryotic. It may be monocentric (if only one) or polycentric if more sporangia are found.

         The hyphae are of two types – substrate hyphae that grow inside the substrate and aerial hyphae that grow on the substrate. Both hyphae are normally branched. The substrate hyphae absorb nutrients whereas aerial hyphae bear reproductive structures. The aerial hyphae very often look black, red, blue, green, grey, or brown, etc. in color due to the spores they produce due to pigments on their hyphal walls.

    The growth of the hyphae is apical.

            Check Your Progress

a. What is septum?

b. What is coenocytic hyphae?

c. Describe the relationship between vegetative structure and reproductive

   cells of unicellular fungi?