Page:The yeasts (1920).djvu/29

 PART I GENERAL

CHAPTER I

MORPHOLOGY AND DEVELOPMENT OF YEASTS

General Characteristics of Yeasts. The Different Phases in Their Development

yeasts are unicellular fungi; generally, they live in isolated states. After they have acquired a certain size, they divide and produce daughter cells which are not slow to separate, enlarge, and divide in their turn in the same manner.

In almost all of the yeasts, cellular division takes place by budding or gemmation. There are a few species from warm climates in which multiplication is effected by transverse division. By reason of this particular method of division, these species have been brought together into a special group and named Schizosaccharomycetaceae.

Budding consists in the appearance, on any side of the cell, of a little bud which slowly increases in size until it ultimately becomes a cell identical with the mother cell.

Partition simply consists of the formation in the middle of the cell of a wall which divides the cell into two daughter cells of equal size which grow eventually.

When division of the cell takes place rapidly, it often happens that many buds are formed simultaneously at different points on the surface, and these daughter cells begin to multiply before separating. This forms a little colony, or conglomeration, of cells. The same phenomenon is observed in the case of the Schizosaccharomycetaceae.

In old cultures and under certain conditions, the cells remain united in long chains; this gives the appearance of a mycelium, but it always remains in a rudimentary state.

The yeasts are then able to present two forms: one, which is most frequent, represents the normal type of vegetation. In this the cells are isolated or united in little groups; the other, which is quite exceptional, is the filamentous or mycelial state.

When the yeast finds, in the medium in which it is cultivated, favorable conditions for growth, it divides actively until the conditions become unfavorable from the lack of food or the accumulation of