ORGANS OF VEGETATION 141 



earth would become a desolate waste without the possibility of 

 vegetable life. The carbon in plants and animals all comes 

 from the air. 



In 1771 Priestley discovered in his experiments that leaves 

 sometimes exhale carbon dioxide and sometimes oxygen, while 

 at other times no gas at all seems to be given off. In 1779 

 Ingenhousz proved that oxygen is given off only when the leaves 

 and the water in which they were immersed were exposed to the 

 sunlight, and that carbon dioxide is given off in the dark. Carbon 

 dioxide, it seems, enters the leaves of the plant by virtue of the 

 forces of diffusion and osmose, and the green parts of the plant 

 have power to decompose this carbon dioxide in such a way that 

 the carbon is retained by the plant while the oxygen is permitted 

 to return to the air. In the entire absence of light plants exhale 

 no oxygen, but only carbon dioxide. In the dark this gas is not 

 only produced within the plant by the action of oxygen upon 

 some portion of it, but is actually given off from the plant into 

 the air. 



The amount of carbon dioxide decomposed at a given time 

 depends on the intensity of the light and also on the kind of light. 

 Experiments show that the decomposition of the carbon dioxide 

 by the leaves is most rapid in yellow light and the least in violet. 

 If the maximum decomposition by yellow light be taken as 100 

 per cent, the relative rate of decomposition of carbon dioxide by 

 the various colors of light will be approximately as follows: 



Violet 7.1 Yellow 100 Orange 63 



Green 37.2 Blue 22.1 Red 25.4 



Indigo 13.5 



Other gases may be absorbed also by leaves, such as the vapor 

 of ammonia or of carbonate of ammonia added artificially to the 

 air, but it does not seem that light in any marked way affects 

 the amount absorbed. The growth of plants is greatly stimu- 

 lated, however, by the presence of either one. 



The air always contains small quantities of invisible vapor of 

 water, and it is contended by some scientists that plants under 

 certain conditions may absorb directly for their growth some of 

 this vapor. This view is apparently borne out by the fact that 



