202 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN. 



upon growth would be by the decrease of the enzymes participating 

 in the various stages of the process. That the rapidity of growth 

 becomes less under illumination in many plants is beyond all doubt, 

 but that such effect is due to the direct paratonic action of radiant 

 energy is a matter that will come up for further discussion in this 

 paper. '*^ The stimulative action of light in chemical processes is well 

 illustrated in the matter of formation and maintenance of chlorophyl. 

 Protoplasm is capable of constructing this complex and unstable 

 substance in darkness, and of maintaining it in a fairly normal con- 

 dition for periods extending over many months. In many species 

 however, the process of formation is not set up except under the 

 stimulation of light, and the entire spectrum appears to participate in 

 the stimulation. Simultaneously, however, the upper end of the 

 spectrum exerts a disintegrating action, which is probably a direct 

 chemical effect of the same character as that by which enzymes are 

 broken down. 



Radiant energy in the form of light being the most important source 

 of energy of plants, it enters into manifold phj^siological relations 

 with the shoot. The plant has coordinately a number of capacities 

 for adjustment to various phases, degree of intensit}^ and angle of 

 incidence of the rays. An added interest is attached to this feature of 

 the subject from the fact that the capacity for these adaptive reactions 

 have been formed to respond to associated characters rather than to the 

 exact portion of the spectrum with which the action of the plant is 

 concerned. Thus the phototropic reactions of plants are induced in 

 greater part by the more refrangible blue violet rays, the resulting 

 movements being for the direct purpose of placing the surfaces of the 

 chlorophyl-bearing organs at a proper angle for the economical and 

 safe reception of the orange red rays. It is true of course that the 

 two kinds of radiations are almost invariably associated so far as the 

 experience of the vegetable world is concerned, but the fact remains 

 that the stimulation in question is one of association. A further ex- 

 ample of such associations in irritability is to be found in the sensi- 

 bility of reproductive organs to light. Seeds and spores are benefited 

 directly in a few instances only, by exposure to light, yet the conditions 

 for the distribution of seeds and spores are more commonly favorable 



'*2 MacDougal. Critical points in the relations of light to plants. A resume 

 read before the Society of Plant Physiology and Morphologj', Baltimore, Dec. 28, 1900. 

 Abstract in Science, 13: 252. 190T. 



