390 ELEMENTARY BIOLOGY. [CHAP. 



The influence of sunlight is an essential condition of the 

 growth and multiplication of Protococcus ; under that influ- 

 ence, it decomposes carbonic anhydride, appropriates the 

 carbon, and sets oxygen free. The energy thus derived 

 from the sun's rays is stored up in the potential form, to be 

 again partly expended in the growth of the plant, and in the 

 movements of its spores. 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 con- 

 tains 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 con- 

 structing 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, of which 

 the chlorophyll forms an essential part. 



