240 CAUSES OF VARIATION 



Light has other influences, however, than those exerted 

 through the fixation of carbon. For example, strong sunlight 

 tends to check growth in the sense of increase in bulk, and when 

 these two effects of light are combined, as they are in the 

 tropics, they give us naturally the slow-growing, generally small, 

 and extremely dense wood of the lower latitudes. 



Briefly, light, like gravity, exerts specific effect upon matter. 

 Many of the effects of gravity (positive geotropism) may be 

 regarded as arising from the elementary properties of matter, 

 for naturally all matter is attracted by, and approaches as nearly 

 as possible to, the surface of the earth ; that is, matter in general 

 may be said to be positively geotropic. Sensitiveness to light, 

 however, should be regarded as due to the special compounds 

 that constitute living matter, rather than as a property of mat- 

 ter in general, for matter in general is indifferent to light. 



Light exerts influence upon living matter, especially plants, in 

 three distinctly different ways : (i) through its heat rays, affect- 

 ing temperatures ; (2) through the so-called chemical (actinic) 

 rays, causing definite chemical reactions in the protoplasm ; 

 (3) through the luminous rays, influencing especially the direction 

 of growth in those parts that are so fixed as to be incapable of free 

 movement. Certain of these influences are worthy of somewhat 

 extended consideration. 



Chemical effects of light. 1 While matter in general in its 

 simpler compounds is quite indifferent to light, yet certain com- 

 pounds are notoriously dependent upon its influence ; that is 

 to say, many combinations are effected more readily and others 

 only in the presence of light (photosynthesis). 



Oxidation of vegetable oils is much more rapid in daylight 

 than in darkness. Hydrogen and chlorin unite explosively in 

 the presence of light. Chlorin passed through alcohol in strong 

 sunlight unites with it, forming chloral hydrate, and chlorin 

 compounds generally are sensitive to light. 



The whole field of photography is dependent upon the action 

 of light upon the halogen salts of silver, gold, platinum, and 

 other metals, due to the so-called "chemical rays" extending 



1 C. B. Davenport, Experimental Morphology, Part I, pp. 161-165, from which 

 most of the instances under this heading are taken. 



