Popular Science Monthly/Volume 21/June 1882/About the Molds

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ABOUT THE MOLDS.

THE molds represent an immense variety of minute plants that grow upon a great number of objects, and under different circumstances. The spores from which they are developed are borne in the air, imperceptibly to us because of their extreme littleness. The microscopic examination of them reveals some very curious dispositions and forms, always worthy of admiration. We often, in the spring, perceive blades of grass covered with a chalk-like dust, so white that one is at first sight inclined to take it to be hoar-frost. On examining it with a microscope of small power, we shall perceive a real forest of

PSM V21 D254 Mycelium.jpg

minute plants. Little bundles of very delicate filaments, clear and crystalline, composed of roundish cells connected together, rise from a network of other branching filaments, which are collectively called the mycelium (Fig. 1). These curious organisms are the first phases of a parasitic plant belonging to the great order of the cryptogams, or a fungus. A mold that has caused great damage to grape-vines—the Oidium Tuckeri—is an incompletely developed fungus.

A whitish substance may be remarked on the leaves of lilacs near PSM V21 D255 Ornamental shapes of mycellium 1.jpg the middle of the summer, which might be regarded on superficial observation as dust from the road. It is also a mold. The microscope shows it to be made up of very delicate threads, similar to those of spiders' webs. As these threads become older, we may observe joined to them a number of little spheroidal bodies, some very small and white, others larger and yellow, and others brown. The white and yellow corpuscles are young fruits of the fungus, and the brown ones are ripened fruits. If one of the last is put into a drop of water and pressed between two glasses for mounting microscopic preparations, it will let escape some small pyriform, transparent sacs, each inclosing spores, the number of which varies according to the species, but is definite for each species. The arrangement of these spore-containing sacs (sporangiæ) is shown by the vertical section (Fig. 4). The number of sporangiæ (b) contained in each of the fruits varies in different species from one to twenty and more.

Aside from every scientific consideration, a great interest is given to these plants by the beauty of their ornamentation; and they form

PSM V21 D255 Ornamental shapes of mycellium 2.jpg

choice objects for preparations. Around each receptacle may be seen numerous appendages radiating in every direction, and generally uncolored. In some genera these appendages are long and flaky, while other genera have only six or eight of them in the form of short needles projecting from a bulbous base (Fig. 3). The extremities of the needles in other species are curved and turned over, as in Figs. 2 and 5, and in others they are one or two times branched; while in the genus Microsphœria the extremities of the appendages take the most varied and most exquisite forms (Figs. 6, 7, 8, and 9).

The life and history of these little plants afford a large field for studies and investigations, which are within the reach of every one who has a good microscope, and is not engaged on any other special study. Such researches are, moreover, of great practical value. The parasitic fungi are one of the great plagues of agriculture. They fix themselves in hosts upon leaves and fruits, where they shut up the stomata and prevent the action of air and light. The hope of discovering a remedy for such evils is dependent on the study of their causes.

 
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