CHLOROPHYLL AND GROWTH 141 



doubt still exists as to the cause of the rapid 

 deterioration of inactive leaves. It is held by 

 some that it is due to the harmful substances 

 set free in the decomposition of chlorophyll, 

 while it is asserted on the other hand that the 

 breaking up of the chlorophyll is a result of 

 starvation and functional inactivity. 



FinalU' it may be said that in order to ac- 

 count for the reactions of inactive leaves in 

 light and in darkness it is necessary to predi- 

 cate the intervention of the regulatory mech- 

 anism in a manner almost entirely specific. 

 Such action has been described in Ariseema, 

 which on the removal of concurrent members, 

 develops laminae in the light and petioles 

 in darkness. That is to say, it should not be 

 taken for granted that all of the changes de- 

 scribed in the above plants are due simply to 

 disturbances of the nutritive processes, but 

 represent to some extent an irritable response 

 of the plant to the unusual conditions under 

 which it is placed and from which it attempts 

 to free itself Thus the excessive elongation 

 of the stems and petioles in darkness seems 

 ver\^ clearl^^ an adaptive modification for lift- 

 ing the leaf-blades and chlorophyll out of ob- 

 scurity and into sunlight, and may not be 

 accounted for on an^^ other grounds. (This 

 theory of elongation as an adaptive modi- 

 fication was originally proposed by Godlew- 



