CHLOROPHYLL AND GROWTH 143 



ness, and be used in such manner as to allow 

 of the perfect development of these organs. 



The removal of concurrent members in dark- 

 ness may have no effect, may cause an exag- 

 gerated development of the petioles, or may 

 result in the perfect development of the entire 

 leaf. The nature of the regulatory mechanism 

 in each instance is entirely specific. 



It is possible for some plants to form perfect 

 leaves in darkness, when some of the branch- 

 es only are darkened, and for others when 

 the entire plant is etiolated. It is thus shown 

 that no invariable connection exists between 

 the phototonic condition and leaf-develop- 

 ment. 



Placing a leaf under such conditions that 

 it cannot construct food sets in motion 

 the specific regulatory mechanism of the or- 

 ganism in such a manner that the plastic 

 material may be withdrawn, and the organ 

 cast off. An exaggerated development of the 

 petioles may be induced in darkness by this 

 mechanism. 



The excessive elongation of the stems and 

 petioles in darkness is to be regarded as a 

 phenomenon of adaptation by which the 

 leaf surfaces would be placed beyond any ob- 

 ject intervening between them and the light. 



