Fig. 37.—A, single cells of yeast. B, C, similar cells, showing the process of budding, × 750.
That the yeast cells are the principal agents of alcoholic fermentation may be shown in much the same way that bacteria are shown to cause ordinary decomposition. Liquids from which they are excluded will remain unfermented for an indefinite time.
There has been much controversy as to the systematic position of the yeast fungi, which has not yet been satisfactorily settled, the question being whether they are to be regarded as independent plants or only one stage in the life history of some higher fungi (possibly the Smuts), which through cultivation have lost the power of developing further.
Class I.—The Smuts (Ustillagineæ).
The smuts are common and often very destructive parasitic fungi, living entirely within the tissues of the higher plants. Owing to this, as well as to the excessively small spores and difficulty in germinating them, the plants are very difficult of study, except in a general way, and we will content ourselves with a glance at one of the common forms, the corn smut (Ustillago maydis). This familiar fungus attacks Indian corn, forming its spores in enormous quantities in various parts of the diseased plant, but particularly in the flowers (“tassel” and young ear).
The filaments, which resemble somewhat those of the white rusts, penetrate all parts of the plant, and as the time approaches for the formation of the spores, these branch extensively, and at the same time become soft and mucilaginous ([Fig. 38], B). The ends of these short branches enlarge rapidly and become shut off by partitions, and in each a globular spore ([Fig. 38], C) is produced. The outer wall is very dark-colored and provided with short spines. To study the filaments and spore formation, very thin sections should be made through the young kernels or other parts in the vicinity, before they are noticeably distorted by the growth of the spore-bearing filaments.
Fig. 38.—A, “tassel” of corn attacked by smut (Ustillago). B, filaments of the fungus from a thin section of a diseased grain, showing the beginning of the formation of the spores, × 300. C, ripe spores, × 300.