GENERAL CONSIDERATIONS. 



Modes of Influence of Light upon Plants. — The relations in which 

 plants stand to radiant energy are so diverse, and the several effects 

 of light and darkness upon plants so intimately interlock that a brief 

 statement of the currently accepted conclusions upon various phases 

 of the subject will be a necessary preliminary to a critical discussion 

 of the records of researches cited in this memoir, and of the new 

 facts which have been brought out in my own investigations. The 

 term light is used in the present paper to denote waves of radiant 

 energy included in the spectrum between the infra-red rays with a 

 wave length of .760 // and the supra-violet with a wave length of 

 .397/i.^^i 



Sunlight has been found to exert analytic, synthetic, isomerismic, 

 polymerismic and catalytic effects upon the chemical substances 

 which may be isolated from the protoplasm of plants. It is fairly 

 probable however that no such extensive action ensues when the 

 various substances and compounds are bound up in the metabolic 

 system of the living cell. At the present time evidence is at hand to 

 show that certain synthetic effects, such as the union of oxygen with 

 some portions of the protoplasmic substances, may be produced in the 

 organism, and that it is to this cause that the destruction of bacteria 

 in sunlight may be ascribed. It has also been found that light exerts 

 a direct influence upon the enzymes in protoplasm. In the earlier 

 stages of such action the effect of the red, orange and some blue rays 

 seems to increase the amount of enzyme present, and later a disin- 

 tegrating effect was exerted by these rays, the violet and ultra-violet 

 being constant in such analytic or catalytic action. To the violet and 

 blue-violet rays is also to be ascribed the oxidizing action noted above 

 as well as the disintegration of chlorophyl. It is of course entirely 

 probable that the action of light may set up chemical processes in 

 the plant is in a manner entirely stimulative, and independent of any 

 communication or transformation of energy. So far as known facts 

 are concerned, the only method by which light might exert an effect 



1*1 MacDougal. Practical Text-book of Plant Physiology, no. 1901. 



