CHAP. II.] CHEMISTRY, ETC., OF PLANT LIFE. 57 



resulting in the formation of starch. Phosphorus in 

 like manner bears a similar relation to the albuminoids^ 

 as these are only formed when phosphates are present in 

 the cells. 



The plant obtains its food from surroundings in a 

 purely physical manner. In the case of submerged 

 aquatics, both carbonic dioxide and all salts are intro- 

 duced in solution in water which is absorbed by the 

 plant. In the case of terrestrial plants, the carbonic 

 dioxide of the atmosphere is taken in by those parts 

 containing chlorophyll, more especially the leaves, accord- 

 ing to the well-known law of gaseous diffusion, which 

 may be briefly expressed as follows : — When two or 

 more gases that do not act chemically on each other 

 are liberated in contact with each other, they gradually 

 diffuse or mix until every portion consists of an equal 

 admixture of all the gases, and when they have thus 

 diffused themselves uniformly through one another, they 

 never separate again in the order of their specific 

 gravities. 



The green parts of plants decompose carbonic dioxide 

 during the day, that is, so long as the chlorophyll is 

 exposed to light; hence the gas that passes from the 

 atmosphere through the stomata into the interior at 

 once loses its individuality. Consequently, obeying 

 the law of diffusion, the gas is constantly passing into 

 the leaf in the attempt to restore equilibrium, and by 

 this purely physical process the plant obtains its carbonic 

 dioxide. During the night, when the chlorophyll can no 

 longer perform its functions, the inflow of carbonic 

 dioxide ceases when the equilibrium between the outer 

 air and the gaseous contents of the leaf is effected. In 



