188 



GENERAL BIOLOGY 



and the production of a sugar, some of which is usually transformed into 

 starch and stored in that form." 



There are mineral substances also necessary for the plant to carry 

 on its life-processes such as magnesium and iron. Nitrogen, potassium, 

 phosphorus, calcium, and sulphur are also required by most plants, al- 

 though it is to be understood that there are very minute quantities of 

 these in so simple a plant as Pleurococcus. 



The process by which proteins and fats are built up is not known 

 in detail, but it is supposed to be due to the action of enzymes. The 

 fats occur in Pleurococcus at those times when the plants become dry 

 and are inactive or in a resting condition. At such times little or no 

 starch is formed while fats are present in quantity. 



YEASTS 



The Pleurococcus just studied, though a simple single-celled plant, 

 is quite complex when compared with a yeast cell. The yeast cell is 

 merely a small mass of granular cytoplasm with various vacuoles scat- 

 tered about. These vacuoles must not be mistaken for nuclei. Often 

 there are little buds (Fig. 87) on the side where a new cell is forming, 



:'-.'?; 

 ViVvf 



Fig. 87. Yeast Cells. 

 n, Nucleus ; v, vacuole ; a, ascms. 



Fig. 88. Various Forms of Bacteria. 



a, Spirillum \ b, Bacillus typhosus ; c. 

 Staphylococcus ; d, e, j, h, Micrococcus ; f, k, I, 

 Bacillus; g, Pauedomonas pycocyanea; i, strep- 

 tococcus. (From G. Stuart Gager's "Fundamen- 

 tals of Botany," by permission of P. Blakis- 

 ton's Sons & Co., Publishers.) 



and sometimes three or four cells will form surrounded by a single wall, 

 in which case the outer wall forms an ascus ( ), 



and the cells contained therein are ascospores. 



The nucleus may be shown by special staining processes. 



Yeasts have been called organized ferments because fermentation is 

 actually associated with the life of the yeast-cell. That is, there are 

 enzymes within the cell (intracellular) which act through the living 

 protoplasm which produced them. They are not poured out as in the 

 saliva or the pepsin (extracellular). 



This power of producing fermentation possessed by yeasts is still 

 retained even though the plant itself be killed with alcohol, ether, or 

 acetone. So, too, the bacteria which cause lactic acid in milk may be 

 apparently killed, thus losing their power to perform any of the normal 

 vital actions such as growing and dividing, and yet be able to produce 

 lactic acid. 



