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. Among the special investigations into the action of 
sunlight we note that of Timiriazeff (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 Bert, by exposing plants 
to the action of light which had been sifted through 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 (A/yagrum sati- 
vum). concludes that the seeds grow in darkness the same as in full 
daylight, and that lght does not seem to influence this stage of 
vegetation. 
Sénébier (1782), 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 water, 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 temperatures under 
different conditions. 
Ingenhousz (1787) exposed an equal number of mustard seeds in 
places receiving different amounts of hght. 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 shows 
that the moisture and the temperature in his several localities varied 
so much as to prevent any serious conclusion as to the action of light 
itself. 
Bertholon (1789), in an article on the effect of electricity, shows 
