308 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN. 



growth when freed from its influence, and the accelerated rate shown 

 by mature organs when placed in darkness may be only ascribed to 

 a stimulative action and an adaptational response. 



Another aspect of the effect of light on growth remains to be 

 considered. It is well established by hundreds of observations that 

 the rate of growth of a large number of normal greenorgans, or of 

 shoots, decreases when deprived of illumination for a period, such as 

 that of an ordinary night, or even briefer, and then suddenly exposed 

 to the action of the rays. This has given rise to the generalization 

 that light exerts a direct or paratonic action on growth. 



Now it has been demonstrated most conclusively that light does 

 not exert any direct effect on the growing region either in the way of 

 influencing cell-division, or the processes depending upon the motility 

 of the protoplasm, or of the material entering into the construction of 

 the membranes. In fact none of the phenomena of etiolation or of 

 diminished growth in light may be ascribed to the direct influence of 

 light upon the tissues or cells concerned but rather upon the organism 

 as a whole. The lessened increase in volume taken into account in 

 measurements of the growth of plants exposed to the action of light 

 may be due to one or both of the following causes. First, it is to 

 be pointed out that the action of the rays on any mass of proto- 

 plasm is to accelerate the rate of transpiration, and the loss of water 

 may be sufficient to cause a decrease in bulk, which might neutralize 

 the outward effects of the actual constructive processes, which may 

 continue uninterrupted during the apparent decrease or cessation of 

 growth. On the other hand, it is entirely probable that some of the 

 apparent retardation may not be due to a direct mechanical influence 

 of the rays, but is a stimulative reaction. That the slowing down 

 of the rate of growth under the influence of light is an irritable 

 response is supported by the behavior of plants exposed to continuous 

 illumination for long periods, such as might occur in the polar re- 

 gions, and which has also been brought about in several series of 

 experiments. In the former instance the specimens grown in locali- 

 ties in which the daylight period embraces the entire vegetative sea- 

 son of several months, the shoot and its members did not exhibit an 

 increase, which either in rate or amount would justify the assertion of 

 a direct retarding influence. The same results have been attained in 

 another form by the exposure of growing plants to continuous ex- 

 posure to electrical illumination, or to an illumination in which day- 



