ORIGIN AND DIFFERENTIATION 549 



the food thus manufactured on the rate and course of development of the 

 plant." 



Dr. Crocker also directed my attention to an important paper by Messrs. 

 Brown and Escombe,^^ in which they record results of the effect of vary- 

 ing amounts of light on photosynthesis. This work was carried on at 

 Kew, England, where so-called full sunlight is not very full, owing to 

 prevalent cloudiness. They say: 



"When a leaf is exposed to full sunshine the radiant energy which is util- 

 ized for the photosynthetic process represents only a very small part of the 

 total incident radiation. If we restrict the term "economic coefficient" to the 

 ratio of these two values, the full radiation falling on the leaf being taken as 

 100, it is evident that the leaf is an extremely wasteful transformer of energy, 

 since it receives a very large amount of superfluous energy which does not 

 contribute to the main function of the leaf. That the photosynthetic rays, 

 even in sunlight of very moderate intensity, are in excess of the power of the 

 leaf to utilize them has been shown by the experiments described in Part 2, 

 XXXX. It was found, for instance, where solar radiation of an average in- 

 tensity of about 0.5 calorie per square centimeter per minute was reduced to 

 about one-third of this intensity by passing through a thin canvas screen, 

 forming an artificial 'cloud,' that it still contained an excess of photosyn- 

 thetic rays over and above what was necessary to produce maximal assimila- 

 tion in ordinary air ; for by means of the revolving-sector method the intensity 

 of the radiation could be still further reduced to one-quarter — that is to- say, 

 to one-twelfth of the original amount — before there was any sensible diminu- 

 tion of the assimilatory power of a leaf submitted to its influence." 



It remains to ascertain the amount of light that would be available for 

 photosynthetic processes under conditions of a complete and permanently 

 overcast sky. These data are supplied in a paper recently published by 

 Dr. H. H. Kimball/^ of the U. S. Weather Bureau. He says : 



"Photometric measurements made at Mount Weather, Virginia, show that 

 with a clear sky the total illumination on a horizontal surface varied from 

 10,000 foot-candles in June to 3,600 foot-candles in January. . . . The illu- 

 mination on a horizontal surface from a completely overcast sky may be half 

 as great as the total illumination with a clear sky, and is frequently one-third 

 as great. On the other hand, during severe thunder-storms at noon in mid 

 summer, the illumination may be reduced to less than one per cent of the 

 illumination with a clear sky." 



These results indicate that the supply of light would undoubtedly bo 

 ample for plant needs, even with a complete cloud mantle. Although 



31 H. T. Browii and F. Escombe : Researches on some of the physiological processes of 

 gi-een leaves, with special reference to the interchange of energy between the leaf and 

 its surroundings. Proc. Roy. Soc. London, Series B, vol. 76, 1905, p. 86. 



32 H. H. Kimball : Photometric measurements of daylight illumination on a horizontal 

 surface at Mount Weather, Va. Monthly Weather Review, vol. 42, 1904, p. 652. 



