THE GENERAL CHEMISTRY AND PHYSICS OF PLANT-LIFE. :i71 



(6) Influence of Light upon Plant-life. 



The difficult problem which we will encounter in this chapter, 

 as well as in the corresponding chapter on gravity, is that the 

 same causes do not produce the same effects in all living cells, 

 but rather produce effects which, to external appearance at 

 least, are directly opposite. The evidence that the principle of 

 adaptation is the controlling factor in such phenomena is so 

 conclusive, that the investigations which stand at the very height 

 of purely scientific and causal-mechanical methods lead to the 

 same conclusion. 



Here also we must speak of a minimum, optimum, and maxi- 

 mum effect of light upon the various organs and life-processes. 

 Elementary jjhysics teaches us that we not only have to deal 

 "vvith the effect of various light-intensities, but also with the effects 

 of different loave-lengths (hence colors). 



In general, chlorophyll-formation requires less light than 

 chlorophyll-activity (assimilation) : the former process, the turn- 

 ing-green of plants, may even take place in the dark ; for exam- 

 ple, in ferns and in the seedlings of conifers. Coloring-sub- 

 stances of flowers may also be formed in the dark, provided the 

 necessary assimilated substances are present, or their formation 

 made possible (Senebiee, de Candolle, Sachs). Sachs further 

 found that the floral buds of Tropceolum require the ultra-violet 

 rays for their development. With the aid of these rays the 

 flower-forming substances would be produced in the green leaves 

 (Sachs). The relation of wave-length to the function of assimi- 

 lation has already been explained (Part II, B). The highest 

 optimum (there are two maxima of assimilation), according to 

 Engelmann, lies in the red spectrum. The rays passing through 

 ammoniacal oxide of copper, hence the highly refrangible rays, 

 are most active in the phenomena of growth and plant move- 

 ment (movement of chlorophyll, heliotropism). They resemble 

 more nearly the activity of normal daylight. 



Effects of Abnormal Illumination and the Conclusions derived 

 therefrom. — Plants which are normally subject to the change of 

 day and night on exposure to continuous darkness show a 

 variety of effects upon the different organs. The pathological 

 phenomena usually depend upon deficient nutritive changes. 



