128 



BACTERIA AND THE CARBON DIOXIDE CYCLE 



enclosing the protoplasmic cell body, which apparently contains a nucleus *. 

 Although very minute, the yeast cells are larger than most bacteria, having 

 on an average a diameter of from 8 ju to 10 /x. All the yeasts that are of 

 industrial importance are colourless, and their cultures appear white or of 

 a yellowish tinge. There is, however, a pink yeast (Saccharomyces glutinis, 

 weakly fermentative) that frequently crops up as an intruder on culture 

 plates, and a less common black species is known. 



Most yeasts under favourable circumstances (free access of air ; surface 

 culture, not immersed ; temperature about 25 C.) form spores. Instead of 

 each cell giving rise to one endospore, as in bacteria, several appear, the 

 protoplasm of the parent-cell dividing into several (generally two to four) 

 rounded masses, each of which becomes surrounded by a new membrane 

 (Fig. 25, g, k}. The spores are considerably less resistant than those of 

 bacteria, being killed by a temperature of 6a-7o C. in five minutes. 

 They are capable of germination as soon as they are formed, but will 

 withstand drying for long periods. The germinating spore begins to bud 

 as soon as the spore-wall is thrown off (Fig. 25, /). 



Hansen's investigations have shown that in the hands of skilled 

 observers who are familiar with the sources of error, the peculiarities of 

 sporulation and its relation to certain temperatures, are of great importance 

 for the differentiation and determination of species and races. 



The following table gives the times of sporulation for a ' high ' yeast 

 (S. cerevisiae\ for a race of wine yeasts (S. ellipsoideus), and for two races 

 of 'wild' yeasts from the air of a brewery (S. Pasteurianus\ Important 

 points are the optimum and the maximum temperatures, and the time 

 occupied in forming the spores. 



* See Wager, ' The Nucleus of the Yeast-Plant,' Annals of Botany, Vol. XII, Dec. 1898, p. 499. 



