12 ELEMENTARY BIOLOGY. [CHAP. 



The influence of sunlight is an essential condition of the 

 growth and multiplication of Protococcus ; under that in- 

 fluence, it decomposes carbonic anhydride, appropriates the 

 carbon, and sets oxygen free. It is this power of obtaining 

 the carbon which it needs from carbonic anhydride, which 

 is the most important distinction of Protococcus, as of all 

 plants which contain chlorophyll, from Torula and the other 

 Fungi. 



As Protococcus flourishes in rain-water, and rain-water 

 contains nothing but carbonic anhydride, which it absorbs 

 along with other constituents of the atmosphere, ammonium 

 salts (usually ammonium nitrate, also derived from the air) 

 and minute portions of earthy salts which drift into it as 

 dust, it follows that it must possess the power of constructing 

 protein by rearrangement of the elements supplied to it by 

 their compounds. Torula, on the other hand, is unable to 

 construct protein matter out of such materials as these. 



Another difference between Torula and Protococcus is 

 only apparent : Torula absorbs oxygen and gives out car- 

 bonic anhydride ; while Protococcus, on the contrary, absorbs 

 carbonic anhydride and gives out oxygen. But this is true 

 only so long as the Protococcus is exposed to sunlight. In 

 the dark, Protococcus, like all other living things, undergoes 

 oxidation and gives, off carbonic anhydride ; and there is 

 every reason to believe that the same process of oxidation 

 and evolution of carbonic anhydride goes on in the light, 

 but that the loss of oxygen is far more than covered by the 

 quantity set free by the carbon-fixing apparatus, which is in 

 some way related to the chlorophyll. 



The still condition of Protococcus, just described, is not 

 the only state in which it exists. Under certain circum- 

 stances, a Protocols becomes actively locomotive. The 

 protoplasm withdraws itself from the cell-wall at all but two 



