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THE AMERICAN NATURALIST. [Vol. XXXVII. 



which finds expression in tissue differentiation. As is pointed out 



that the plant having adapted itself to this condition ceases to 

 respond so readily to the stimulating action of light. It may be 

 narrowed down then to the fact that it is light which acts as the 

 stimulating influence in inducing morphological differentiation and in 

 its absence the tissues tend to remain in a more or less primitive con- 

 dition. The action of light, however, is not necessarily direct, since 

 the stimulative influence may be received by one portion of the body 

 and transmitted to another, as is shown by local aetiolation. The 

 author speaks of the enlargement of certain dorsiventral leaves 

 brought about in etiolated plants by the removal of concurrent 

 organs and mentions that even in their enlarged condition they do 



son for this behavior. Possibly this might be regarded as a reaction 

 to the untoward stimulus of wounding. 



The total amount of growth or increase in volume, that may be 

 accomplished by the shoot in the expansion of its imperfectly devel- 

 oped tissues during etiolation, is subject to great variation. In 

 many cases the total length, diameter, and volume may be actually 

 less than in a normally grown plant, and indeed the rate of growth 

 need not be so rapid. It may be said that light does not directly 

 affect the rate of growth, that it does not exert the retarding or para- 

 tonic influence commonly ascribed to it. This is shown by the action 

 of plants exposed to continuous illumination. 



of the influence of light on the growing shoot. Never before has 

 this point of view received such a comprehensive treatment or been 

 supported by such a wealth of facts. The author's insistence that 



anatomical conditions presented by etiolated organs, leads to a far 

 clearer understanding of the actual influence of light as a morpho- 

 genetic stimulus. In this latter regard the evidence afforded is 

 especially convincing. It is not yet clear of course why light should 

 so affect tissues as to practically shorten the period of their meriste- 

 matic condition and to induce the formation of what is usually termed 

 " permanent " tissue. It is said that the aplastic material is not so 

 readily laid down in the absence of light, a statement which suggests 

 a chemical explanation. The exact nature of the chemical changes 

 which are supposed to take place in the differentiation of the usual 



