30 LIFE AND LIGHT 



grown under otherwise like conditions in ordinary air, assumed 

 a stunted and bushy appearance. Lastly, and perhaps most im- 

 portant of all, the reproductive system became entirely aborted, 

 not a single flower in any case arriving at maturity. 



It is thus obvious that excess of carbon dioxide possesses a most 

 potent influence upon cell division and regulation of growth in plant 

 cells, similar to that which has been noted as obtaining in animal 

 cells in the previous chapter. 



Somewhat divergent results also exist amongst the records as to 

 the effects of variations in light intensity upon speed of photo- 

 activity. All authors agree as to the existence of an optimum 

 intensity of illumination, but some place it at full sunlight, others 

 at half or quarter of full insolation, finding little or no increase, or 

 even elimination, as the light intensity is increased beyond the 

 observed optimum. Again, in the region of intensities where 

 admittedly increasing illumination causes increased synthesis, 

 some authors find a direct proportionality of linear type, while 

 others find the photo- synthesis advancing much more slowly than 

 light intensity. 



Thus, Pantanelli 1 found the optimum assimilation at one-fourth 

 of full sunlight, and a diminution at greater intensities. Kreusler 

 found, when experimenting with electric arc light, that the action was 

 roughly proportional to the light intensity, but Brown, experiment- 

 ing in sunlight, found the uptake of carbon dioxide only doubled, 

 while the light intensity increased twelvefold. Treboux and also 

 Pantanelli discovered that the optimum intensity of light and op- 

 timum concentration of carbon dioxide varied together, and this 

 may account for some of the discordance between different authors 

 thus, with one-fourth the intensity of direct sunlight, maximum 

 photo- synthesis corresponded to 10 per cent, by volume of carbon 

 dioxide, with full sunlight to 15 per cent, of carbon dioxide, and 

 fourfold sunlight produced by a converging system yielded the 

 maximum effect with 20 per cent, of carbon dioxide. 



It is obvious that the amount of photo- synthesis occurring at 

 various periods of the day will vary with the intensity of the light, 

 but when the light is artificial and kept continuous and constant, 

 it is found that there is no habit corresponding to the natural 

 alternating diurnal activity and rest of the chloroplasts, but the 

 activity with equal artificial illumination is about the same in the 



1 For references see Czapek, "Biockernie d. Pflanzen," vol. i., p. 434. 



