MUCOR: A SAPROPHYTIC FUNGUS

Mucor is a ’mould ’that is commonly found growing saprophytically on moist organic matter. If a piece of bread is moistened and kept under a bell-jar for a few days it will soon become covered with a growth of Mucor, which appears as a closely woven mass of white silky threads. This mesh of threads is the vegetative part or mycelium of the fungus, and the hyphae of which it is composed are non-septate branched tubes. Each hypha contains cytoplasm in which there are many nuclei, and has a cell wall.

The hyphae penetrate the dead matter on which they spread, and secrete enzymes ; these pass through the cell walls of the hyphas and convert the organic matter into soluble substances which can be absorbed by the fungus. In this way Mucor obtains nutriment from the matter on which it grows.

Reproduction Asexual Reproduction

A few days after the appearance of

Mucor on a piece of bread, little black dots at the end of upright hyphaj will be seen on the mycelium. These are the asexual reproductive organs or sporangia, each of which contains many small spores. The spor angia are formed by the tips Black wall of the erect hypha? enlarging to form spherical swellings.

A transverse wall is formed across the base of each swell ing whose contents develop into a large number of spores.

As spore formation proceeds, the transverse wall protrudes upwards into the interior of the sporangium to form the columella and the wall of the sporangium darkens and becomes impregnated with crystals of calcium oxalate. When the sporangium is ripe it bursts and the spores are set free. These are very light and easily blown about by the wind, and germinate readily if they fall on some suitable clamp organic matter. The germinating spore puts out a hypha which soon branches and develops into a mycelium. Sexual Reproduction

Occasionally Mucor reproduces by a sexual method. The tips of two hypha? become swollen and come together so that they touch. A cross wall appears at the base of each bulge, enclosing several nuclei in each bulge, which is now known as a gametangium. Shortly afterwards the end walls dissolve, and the nuclei fuse in pairs, one from each gametangium fusing with one from the other. An outer covering now forms. This thickens and darkens, becoming warty on the outside. This black body is a zygospore, and is able to remain dormant for some time, being resistant to drought. If it is blown to a moist spot where there is nourishment, it germinates. The case cracks and a short aerial hypha grows out, producing a ripe sporangium about twenty-four hours after germination began, so that dispersal and perpetuation of the species can occur rapidly whilst the good conditions last.

In some species of fungi allied to Mucor any two hyphze can conjugate, but in Mucor itself conjugation can only occur between hyphas of two different strains which are exactly similar to look at and are therefore structurally the same, but their protoplasm must be physiologically different. When the zygospore of such a species germinates and produces a sporangium, the spores from this sporangium are also physiologically different, some being of one strain and some of the other. In the majority of cases the masses which fuse together are similar in shape, size and function, and are clearly isogametes. Since they are so much alike we usually designate the strains as being + and —, unlike the functional differentiation exhibited by the gametes of Spirogyra. It is important to note, however, that only a part of a Mucor plant functions as a gamete, and that the zygospore is formed as a result of the fusion of several pairs of nuclei.

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