42 



of the light and the action of the chlorophyl of the leaves. As we 

 pass from the pole to the equator the luminous intensity of the sun- 

 light increases from a hundred to a thousand, but its duration dimin- 

 ishes during the growing season from a hundred at the poles to fifty 

 at the equator. ^Vmong the special investigations into the action of 

 sunlight Ave note that of Timiriazelf (1877), who has shown that a 

 very intense light, after traversing a certain thickness of green-leaf 

 cells, has no further action on the phenomena of the reduction or 

 decomposition of carbonic-acid gas; in other words, it acts the same as 

 darkness would do. On the other hand, Paul 1 jert, by exposing plants 

 to the action of light which had been sifted tlirough a solution of 

 chlorophyl, invariably found that the development of the green mat- 

 ter of the leaf was completely arrested; inversely, he found the green 

 matter produced to its normal amount when the plant received only 

 light that had been filtered through a solution of iodine in bisulphide 

 of carbon, which solution, as we know, cuts off all visible rays, but 

 allows the red and infra-red to pass through with great freedom. 

 This would seem to demonstrate that chlorophyl is formed by the 

 action of the red portion of the spectrum. 



As to the effect of light on the germination of seeds, Pauchon 

 (1880) gives a critical summary of views by different authors, from 

 which we condense the following : 



Miesse (1775), from observation on the Camelina {Myagricni sati- 

 vum), concludes that the seeds grow in darkness the same as in full 

 daylight, and that light does not seem to influence this stage of 

 vegetation. 



Senebier (178'2), from observations on seeds of lettuce and beans, 

 some of which were exposed to the full sunlight, others to sunlight 

 after filtering through a thickness of Avater, others in the dark, and 

 others in red, violet, and yellow light, respectively, reached the 

 conclusion that light was injurious; but his results were not decisive, 

 because of his neglect to observe exactly the temperatiu'es under 

 different conditions. 



Ingenhousz (1787) exposed an equal number of mustard seeds in 

 places receiving different amounts of light. He himself concluded 

 that the light of the sun is as injurious to vegetation at the beginning 

 of its life as it is advantageous to vegetation in the fullness of its life. 

 But a more careful consideration of Ingenhousz's experiments shoAvs 

 that the moisture and the temperature in his seA^eral localities varied 

 so much as to preA'ent any serious conclusion as to the action of light 

 itself. 



Bertholon (1789), in an article on the effect of electricity, shows 



