Page:The yeasts (1920).djvu/30

 products of metabolism; it ceases, then, to divide and produces organisms which allow it to perpetuate itself over unfavorable conditions.

Will and Casagrandi, under these conditions, have observed cells filled with reserve products (fats and glycogen) enclosed in a thick wall. They offer a great resistance thanks to these reserve products, which they retain for a long time during suspended activity, until favorable conditions allow them to develop again. The cells, which are comparable to cysts, have been designated under the name of "durable cells."

But the ordinary process employed by the yeasts for perpetuation of the species is sporulation; a certain number of internal, or endospores, are formed in the interior of each cell. The cells are thus transformed into a sort of sporangium which is called an asc. The spores or ascospores formed in these ascs are endowed with a great resistance against external conditions and during years of suspended activity. When placed in favorable conditions, they swell up and rupture the asc wall to become free. They then offer the appearance of vegetative cells and multiply in the ordinary way.

In certain species, the formation of the asc is preceded by a sexual process; the asc then results from the fusion of two cells -- a copulation as in the case of an egg. In other species, sexuality is maintained in a lower state of development; in this case, it takes place between two spores at the moment of germination. In the greater number of yeasts, however, no sexuality has been observed.

Many of the yeasts, as Torula and Mycoderma, do not form endospores. We shall investigate successively, in this chapter, the morphological characteristics of yeasts: the form and shape of the cells, mycelial formations, durable cells, cellular division, formation of the asc, sexuality, and germination of ascospores.

Forms of Cells

The yeasts offer forms varying usually from a sphere to an ellipse. They possess quite a thick membrane. The greater number of them have a colorless interior containing vacuoles and refractive granules. Often, a red pigment may be observed, sometimes a brown, gray or yellow one ; in this case it is probably not a true Saccharomyces but a yeast without endospores. However, Hansen has observed a rose-colored yeast which did produce endospores. The dimension of yeast cells varies between from 1 to 4 or 5 μ in width and from 1 to 5 or 9 μ in length. There is a great difference in the cells of the same species. The yeasts are very polymorphic and are capable of assuming