12 COMPARATIVE PHYSIOLOGY. 



Like Torula it is selected because of its simple nature, its 

 abundance, and the ease with which it may be obtained, for it 

 abounds in water-barrels, standing pools, drinking-troughs, 

 etc. 



Morphological. — Protococcus consists of a structureless wall 

 and viscid granular contents, i. e., of cellulose and protoplasm. 



The protoplasm may contain starch and a red or green color- 

 ing matter (chlorophyT). It probably contains a nucleus. The 

 cell is mostly globular in form. 



Physiological. — It reproduces by division of the original cell 

 (fission) into similar individuals, and by a process of budding 

 and constriction (gemmation) which is much rarer. Under the 

 influence of sunlight it decomposes carbon dioxide (OOa), fixing 

 the carbon and setting the oxygen free. It can flourish per- 

 fectly in rain-water, which contains only carbon dioxide, salts 

 of ammonium, and minute quantities of other soluble salts that 

 may as dust have been blown into it. 



There is a motile form of this unicellular plant, and in this 

 stage it moves through the fluid in which it lives by means of 

 extensions of its protoplasm (cilia) through the cell wall ; or 

 the cell wall may disappear entirely. Finally, the motile form, 

 withdrawing its cilia and clothing itself with a cellulose coat, 

 becomes globular and passes into a quiescent state again. 

 Much of this part of its history is common to lowly animal 

 forms. 



Conclnsloiis. — It will be seen that there is much in common 

 in the life-history of Torula and Protococcus. By virtue of 

 being living protoplasm they transform unorganized material 

 into their own substance ; and they grow and reproduce by 

 analogous methods. 



But there are sharply defined differences. For the green 

 plant sunlight is essential, in the presence of which its chloro- 

 phyl prepares the atmosphere for animals by the removal of 

 carbonic anhydride and the addition of oxygen, while for 

 Torula neither this gas nor sunlight is essential. 



Moreover, the fungus (Torula) demands a higher kind of 

 food, one more nearly related to the pabulum of animals ; and 

 is absolutely independent of sunlight, if not actually injured by 

 it ; not to mention the remarkable process of fermentation. 



