6 MORPHOLOGY AND DEVELOPMENT OF PLANTS 



products of metabolism; it ceases, then, to divide and produces or- 

 ganisms which allow it to perpetuate itself over unfavorable con- 

 ditions. 



Will and Casagrandi, under these conditions, have observed cells 

 iilled 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 endo- 

 spores, are formed in the interior of each cell. The cells are thus trans- 

 formed 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 vegeta- 

 tive 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 endo- 

 spores. We shall investigate successively, in this chapter, the mor- 

 phological 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 l has observed a 

 rose-colored yeast which did produce endospores. The dimension of 

 yeast cells varies between from 1 to 4 or 5 ^ in width and from 1 to 5 

 or 9 /x in length. There is a great difference in the cells of the same 

 species. The yeasts are very polymorphic and are capable of assuming 



1 Hansen, E. C. 1879. Comp. Rend, des trav. du lab. de Carlsberg, 24. 



