Intermediate Organisms 



189 



use of this energy brings about a chemical synthesis of the materials, 

 resulting in the freeing of oxygen and the production of a sugar, some 

 of which is usually transformed into starch and stored in that form." 



Some mineral substances, such as magnesium and iron, are also 

 necessary for the plant to carry on its life-processes. Nitrogen, potas- 

 sium, phosphorus, calcium, and sulphur are also required by most plants, 

 although 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, 



Fig. 87. 



n, Nucleus; i 



Yeast Cells, 

 vacuole; s, ascus. 



Fig. 88. Various Forms of Bacteria. 



a, Spirillum; b, Bacillus typhosus; c, 

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

 Bacillus; g, Psuedomonas 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 within 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 



