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 weights. 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 
oxygen 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, 
but assimilation is a process peculiar to the plant life. 
By transpiration 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 
(67) 
