42 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



passage of only one kind of light. This falls upon a leaf placed 

 in water containing carbon dioxide. All the colors except the one 

 to be tested are excluded by the screen. 1 The number of bubbles 

 of oxygen liberated from the leaf in a given time is taken as a 

 measure of the action of the light in producing organic matter by 

 the decomposition of carbon dioxide. More accurate results are 

 secured if the volume of oxygen is measured. 



Control of Light. Only in isolated cases is control of light of 

 practical significance in agriculture. Forcing of early vegetables 

 by artificial light has been tried but has not proved successful 

 enough to be generally adopted. Cigar wrapper tobacco is grown 

 under the shade of cheesecloth or slats. 2 Reduction of light by 

 shading makes the plant grow taller and produce thinner leaves 

 than under ordinary conditions. The thin leaves bring high 

 prices for use as wrappers in making cigars. The shading, how- 

 ever, also modifies moisture and temperature conditions. 3 



Oxygen. Oxygen is necessary to both animal and plant life. 

 Without oxygen, animals quickly die from suffocation. The 

 oxygen is required by animals for processes of oxidation 

 necessary to life, such as the production of animal heat. The 

 oxygen consumed is replaced by carbon dioxide in the respired 

 air. Oxygen is also consumed in the decay of organic matter, in 

 combustion, and in other processes of oxidation. 



The oxygen lost from the air by oxidation is restored by green 

 plants, which, as we have seen, absorb carbon dioxide and evolve 

 oxygen. On account of diffusion and air currents, the quantity 

 of oxygen in the air varies but slightly. In analyses made in 

 widely separated parts of the world, the minimum and maximum 

 amounts of oxygen in pure dry air are 20.53 an< ^ 2I -3 parts by 

 volume. 



Oxygen is necessary for the germination of seeds, for the 

 development of buds, for the roots of certain plants, and for 



1 Pfeffer, Jahresber Agr. Chem., 1870-2, p. 179. 



2 Report No. 62, U. S. Dept. Agr. 



3 Stewart, Bui. 39, Bureau of Soils. 



