72 THE PlyANT WORLD 



processes. " It is to be seen therefore that the phenomena of etiolation 

 rest upon, and consist in the behavior of the plant consequent upon the 

 absence of the morphogenetic influence of light." This conclusion is 

 still further emphasized in the chapter treating of the illumination of 

 etiolated plants, where the effect of light upon etiolated specimens of 

 Arisaema, Asplenium, Peltandra, etc., is shown to be truly morphogenetic. 



The stimulative influence of light which produces morphological dif- 

 ferentiation is found to be due not to any direct effect upon any particular 

 organ or tissue, but that the stimulus ma}^ be received and transmitted 

 from one part to another, even being communicated to organs not yet 

 formed. The only conclusion that can be drawn with regard to the 

 differences observed in continuous and discontinuous light is that the 

 stimulus necessary for morphogenetic action is in the alternate effect of 

 light and darkness, rather than in light itself. 



The influence of etiolation upon chemical composition is considered 

 and the final chapter of the memoir is concerned with the rate and mode 

 of growth as affected by light and darkness. That light retards growth 

 has been accepted as a well-established fact for many years, there being 

 many scientific investigations, as well as much popular experience sup- 

 posed to justify this view. While it is true that certain plants do exceed 

 their normal growth in the absence of light, the fact that the reverse is 

 quite as true, together with other considerations, has led Dr. MacDougal 

 to state as the result of his numerous observations that ' ' light has no 

 invariable and universal relation to increase in length or thickness or to 

 the multiplication or increase in volume of the separate cells." The ex- 

 planation of the frequently observed fact that plants decrease in growth 

 when brought into the light after even a short period of darkness, is to 

 be found either in the loss of water due to increased transpiration, which 

 might effect a decrease in bulk sufficient to offset the actual growth, or 

 more probably to an irritable response to which the plant gradually ac- 

 commodates itself when subjected to continuous illumination. 



It is hoped that the foregoing brief and inadequate summary of Dr. 

 MacDougal 's book may be sufficient to indicate in a general way the 

 scope of the work and that it will lead to a careful examination of such 

 admirable contribution to this aspect of plant physiology. Probably it 

 would have been an assistance to some if there had been a chapter devoted 

 to a general summing up of all the conclusions reached by the author, 

 as certain points are apt to be passed over by any but the most careful 

 reader. However, the most excellent figures in the text, of which there 

 are some 178, practically all of which are original, serve in a graphic way 

 to point out and corroborate the more important principles established. 



George T. Moore. 



