182 



UNICELLULAR PLANTS. 



quantities of which are dissolved in the water, and other neces- 

 sary salts (sulphates, chlorides, phosphates, etc.) as well as free 

 oxygen are procured from the same source. These substances 

 may be derived from dust blown or washed by the rain into the 

 water, or from the walls of the vessel. To the process of starch- 

 making, attended by the absorption of CO, and H,O and the 

 liberation of O, the term ' ' assimilation ' ' is generally given. 

 Like other plants, moreover, Protococmis probably breathes 

 by absorbing free oxygen and setting free CO, (respiration). 



The income and outgo of Protococcus may then be displayed 

 by the following diagram : 



II 2 



FreeO 



lncojne 



Outgo 



It should be understood that this only represents the broad 

 outlines of the process and under the simplest conditions. It is 

 quite possible that under other conditions Protococcus may use 

 more complex foods. The facts remain, however, (1) that 

 Protococcus is dependent on the energy of light; (2) that its 

 action is on the whole constructive, resulting in the formation of 

 complex compounds (carbohydrates, proteids) out of simpler 

 ones. In these respects it shows a complete contrast to Am&ba, 

 which is on the whole destructive, breaking down complex com- 

 pounds into simpler ones, and is independent of light, since it 

 derives energy from the potential energy of its food. The 

 relations between Protococcus and Amoeba are therefore an 

 epitome of the relations between Pteris and Lumbricus, and 

 between green plants and animals generally. 



The Fundamental Physiological Properties of Plants. In con- 

 sidering the physiology of Amoeba we found it possible to re- 



