ADVERTISEMENTS:
In this article we will discuss about the somatic phase of rhizopus stolonifer. This will also help you to draw the structure and diagram of rhizopus stolonifer.
Thallus (Fig. 7.1):
It consists of numerous, slender, freely branched filaments, the hyphae. The hyphae are tangled and form a fluffy, white mass which makes up the thallus of the fungus. It is called the mycelium (Fig. 7.1).
ADVERTISEMENTS:
It is cottony white during the vegetative phase. However, it soon enters the reproductive phase. Numerous black, pin head-like structures develop on the mycelium giving the entire cowebby mass a blackish appearance, hence the popular name black molds.
Are all hyphae constituting the mycelium alike? During the early period of growth the mycelium consists of hyphae which are all alike. They remain within the substratum. After a few days of growth this internal mycelium produces some hyphae which come to the surface of the substrate to form an aerial mycelium. The latter shows differentiation of hyphae accompanied by division of labour.
It consists of three lands of hyphae:
(i) Stolons:
ADVERTISEMENTS:
These are aerial hyphae that grow horizontally over the surface of substratum. They are comparatively stouter and of larger diameter, slightly arched and less branched aerial hyphae (Fig. 7.1). The stolons grow rapidly in all directions and cause further spread of the mycelium over the substratum.
Each stolon arises from the point of contact of mycelium and substratum. It runs horizontally for some distance and then arches over so that its tip touches the substratum further along to form an apparent node-like structure. From the latter arise fresh stolons which run over the surface of the substratum and repeat the process.
(ii) Holdfast:
It is a cluster of brown, slender, branched rooting hyphae (Fig. 7.1) which arise from the lower surface of the apparent node of each stolon and penetrate the starchy substance of the substratum. They secrete, as they grow down, an enzyme which digests starch. The rooting or rhizoidal hyphae thus serve a double purpose.
They anchor the fungus to the substratum and absorb water and nourishment from it for the entire plant. During the vegetative phase the mycelium consists only of these two kinds of hyphae and is cottony white. It becomes moldy as it enters the reproductive phase and develops the third kind of hyphae.
(iii) Sporangiophores:
Just opposite each holdfast one or more vertically growing hyphae rise into the air (Fig. 7.1). They are reproductive in function and bear sporangia singly and terminally. These negatively geotrophic, unbranched special hyphae are called the sporangiophores.
Structure of Thallus (Fig. 7.2):
ADVERTISEMENTS:
The hyphae which collectively constitute the mycelium are transparent, tubular structures. Each hypha has a limiting hyphal wall which consists of a substance called fungus chitin and not cellulose. Besides hydrogen and carbon it contains nitrogen.
In electron micrographs it is seen to be microfibrillar in structure with the microfibrils running R-Stolonifer. A hypha showing detailed structure as seen under light parallel to the surface. The chitinous wall of Rhizopus like the cellulose wall of green algae is permeable to water and other substances in solution.
Electron microscope studies reveal the presence of a thin plasma membrane lining the hyphal wall. It closely invests the hyphal protoplasm and regulates the movements of substances in solution into and out of the hypha.
ADVERTISEMENTS:
The hyphal protoplast is granular and contains numerous minute nuclei, glycogen and globules. The young hyphal tips are full of dense protoplasm. Further back, in addition, occur small, sap-filled vacuoles which in the older portion coalesce to form a central vacuole.
Embedded in the cytoplasm are numerous, small ribosomes and a branching system of tubes constituting the endoplasmic reticulum. The ribosomes are of uniform size. Mitochondria with double membranes and cristale also occur in the cytoplasm. The nuclei are bounded by a two-layered nuclear membrane with pores.
The cross walls remain suppressed in the actively growing hyphae. The protoplasm is, therefore, not divided into cells. It is continuous throughout the entire mycelium. Such a plant body or a portion of it, in which a multinucleate condition arises by the division of nuclei un-accompanied by wall formation, is said to be coenocytic.
ADVERTISEMENTS:
The hyphae in Rhizopus are thus aseptate and coenocytic. The cross walls are, however, found in the old hyphae and in connection with reproductive structures. These septa are solid plates. The growth of hyphae is entirely apical.
Nutrition:
Rhizopus lacks chlorophyll. Consequently it is unable to synthesize its organic food from carbon dioxide and water. How does then bread mold gets its nourishment? It gets its food readymade from the dead organic matter present in the substratum by means of the root-like, rhizoidal hyphae.
The latter secrete enzymes as they penetrate the substratum. The enzymes convert insoluble starch and other substances into a soluble form. The solubles enter the rhizoidal hyphae by diffusion through the hyphal walls.
ADVERTISEMENTS:
In its mode of nutrition bread mold thus is a heteromorph and a saprophyte.
Although the fungus is unable to manufacture sugar from carbon dioxide and water, yet it is able to convert the soluble sugars it absorbs from an external source into complex carbohydrates which are the chief components of the hyphal walls. It can also synthesize proteins from the carbohydrates utilizing inorganic nitrogen compounds such as ammonium salts.
