Chapter IV. 



THE INFLUENCE OF SUNSHINE ON ASSIMILATION AND TRANS- 

 PIRATION. 



CHEMISTRY OF ASSIMILATION (ABBOTT). 



The atmosphere is composed of about 79 per cent of nitrogen and 

 21 per cent of oxygen when we consider their volumes, but 77 per 

 cent of nitrogen and 23 per cent of oxygen when we consider their 

 relative Aveights. With these gases there are mixed small quantities 

 of carbonic-acid gas, ammonia, hydrocarbons, and other impurities. 

 With this '' dry atmosphere " there is intermixed a very variable quan- 

 tity of aqueous vapor or moisture, which in extreme cases may amount 

 to as much as 5 per cent, by weight, of the dry air. These are the 

 elements that are to be compounded by sunshine and heat in the 

 laboratory of vegetation. 



By respiration the leaves of plants, when in the dark, absorb 

 ox3'gen from the air and set free carbonic-acid gas. 



By assimilation, as shown by Garreau, these same leaves in the 

 sunshine absorb carbonic-acid gas from the air and set free oxygen, 

 retaining the carbon in new compounds. Assimilation is a process 

 of greater intensity than respiration. Respiration is a process analo- 

 gous in its results to that occurring within every animal organism, 

 l)ut assimilation is a process peculiar to the plant life. 



By transjDiration the leaves rid themselves of the superfluous water 

 that, as sap, has served its purpose in the process of assimilation by 

 bringing nourishment from the soil and delivering it up to the cells 

 of the plant ; a small portion of the nourishment and of the water 

 may have been absorbed by the cells in the trunk of the tree, the stem 

 of the vine, or the stalk of the grain and grass, but the majority of 

 the water is removed by transpiration at the surface of the leaves in 

 order to make room for fresh supplies of sap. Some water always 

 remains in the cells of the seeds and grains until they are dried after 

 maturity, but a well-dried crop contains relatively little water. This 

 transpiration is stimulated by, and almost entirely depends upon, the 

 action of sunshine on the leaves; it precedes evaporation. 



Evaporation is not transpiration; the former takes place from the 

 surface of water existing either in the moist earth or in films on leaf 

 surface or in larger masses, while transpiration takes place through 

 the cell wall and is a process of dialysis, an endosmosis and exosmosis 



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