12 ELEMENTARY BIOLOGY. [ll. 



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 rearrangment of the elements supplied to it by 

 their compounds. 



Torula, on the other hand, is unable to construct protein 

 matter out of such materials. Another difference between 

 Torula and Protococcus is only apparent : Torula, absorbs 

 pxygen and gives out carbonic 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 anhy- 

 dride; 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 circumstances, 

 a Protococcus becomes actively locomotive. The protoplasm 

 withdraws itself from the cell-wall at all but two points, 

 where it protrudes through the wall in the form of long 

 vibratile filaments or cilia, and by the lashing of these cilia 



