GENERAL 



33 



such plants may not absorb any organic nutriment even from a soil rich in 

 humus (Sects. 50, 51, and 64). Indeed green plants produce much more 

 organic material than can possibly be harvested, for a large part is 

 decomposed again in respiration, or is lost by the fall of old leaves or 

 dead parts. 



The assimilation of carbonic acid is possible only by means of the 

 radiant energy of the sun, and it ceases instantly in darkness, so that only 

 those chlorophyll bodies are functionally active which are exposed to 

 light, and hence chloroplastids are usually found only in the subaerial 

 organs, and especially in the leaves. An adequate supply of carbon dioxide 

 and an optimal intensity of light are necessary for the full functional activity 

 of the chloroplastids. The atmosphere contains mere traces of carbon 

 dioxide, and hence various adaptations have arisen by means of which rapid 

 gaseous exchange is rendered possible, for almost the whole of the vast 

 quantity of this gas which a green leaf may assimilate comes directly 

 from the air. The same applies to an aquatic plant, for the water usually 

 contains but little carbon dioxide, though relatively more than the air. 



In a confined space the supply of carbonic acid is extremely limited, 

 and is soon used up, while every morning the respiratory carbon dioxide 

 which has accumulated during the night suffices to allow the chloroplastids 

 to exercise their normal functional activity for a short time. If, however, 

 the exhaled carbon dioxide is continually absorbed by a solution of potash, 

 the dry weight of the plant gradually decreases just as it would do if 

 kept continually in darkness. 



Since respiration necessarily accompanies all vital activity it must 

 also continue without cessation in all cells which are assimilating carbonic 

 acid. This is shown by the fact that growth and streaming movements are 

 still exhibited when a green cell is exposed to light in an atmosphere of 

 hydrogen. It is possible to prove directly that both green and non- 

 green cells evolve carbon dioxide acid without cessation, both in the 

 light and in darkness (Chap. IX), although the chlorophyllous cell when 

 exposed to light may, as the resultant of the two processes of respiration 

 and assimilation, excrete oxygen and absorb carbon dioxide from the 

 surrounding air. In darkness green plants respire undisturbedly, and hence 

 there must be a certain intensity of illumination at which the composition 

 of the surrounding air remains unchanged. 



From what has been said above it is evident that the assimilation of 

 carbonic acid is simply a special means by which a supply of organic food 

 is ensured, and that this food has the same importance in the metabolism 

 of the plant as has similar organic food absorbed from without. This 

 organic food, from whatever source it is derived, must be distributed to 

 all parts which consume it, and hence the starch-grains which may 

 appear in the chloroplastids of many plants when assimilation is active, 



