In this article we will discuss about the morphological characteristics of fungi with the help of diagrams.
The slime-molds are morphologically distinct from other fungi in having a body consisting of either cell wall-less amoebae (cellular slime molds e.g. Dictyostelium) or a mass of multinucleate protoplasm in which individual cells are indistinguishable (acellular slime molds e.g. Stemonitis, Ceratomyxa etc.). Other fungi have either single cells e.g. yeasts, chytrids etc., or mycelia.
Unicellular forms may be motile or non-motile. Mycelial fungi can have septate or aseptate hyphae. In unicellular and hyphal fungi, the cell is externally bound by a firm but elastic cell wall composed of micro fibrils of cellulose, chitin or other polymeric compounds. The micro fibrils are embedded in a matrix of proteins, lipids and other substances. Chitin is a characteristic component of the cell wall of most higher fungi. It is a polymer of N-acetyl glucosamine in which the monomers are linked to each other by 1, 4-β- glycosidic bonds (Fig. 5.1). Interestingly, chitin is also present in arthropods.
The protoplast of fungal cells is typically eukaryotic containing membrane-bound nucleus and other cell organelles, like mitochondria, rough and smooth endoplasmic reticulum, microtubules, Golgi bodies etc. Ribosomes are of 80S type. Vacuoles are often present occupying the major part of the cells, pushing the cytoplasm to the periphery. Cytoplasmic streaming is also observed. As reserve material, fungi generally accumulate glycogen which is a branched polymer of glucose. Fats and oils are also often present.
In the majority of fungi, the vegetative body is made of hyphae. A hypha generally originates by germination of a spore which may be produced by asexual or sexual means. The germ-tube of a germinating spore elongates into a hypha which grows at or near the tip.
Hyphae of lower fungi, like water-molds and oomycetes are broader, non-septate, multinucleate and coenocytic in vegetative stage. In contrast, hyphae of higher fungi—like ascomycetes and basidiomycetes—are less broad, septate and contain generally one or two nuclei per cell.
The septum arises by centripetal growth of the hyphal wall, but the inward growth of the septum remains incomplete leaving one or, sometimes, more than one gap, called a pore through which contact between the two adjacent cells is maintained.
Sometimes, individual hyphae grow intertwined with each other to form a more or less compact tissue, called plectenchyma. When, in such a tissue, the individual hyphae are recognizable, it is known as prosenchyma. On the other hand, the tissue is called a pseudo-parenchyma when the individual hyphae lose their identity.
In some fungi, the pseudo-parenchymatous tissue may form a small tuber-like structure, called a sclerotium. Sclerotia may be spherical, as in Sclerotium rolfsii or an elongated structure, as in Claviceps purpurea. In some fungi, the hyphal tissue may form an elongated, branched root-like structure, known as a rhizomorph, as in Armillaria mellea. Generally, the complex hyphal tissues are found in higher fungi. Some morphological features of fungi are depicted in Fig. 5.2.