﻿THE DEVELOPMENT OF IRRITABILITY 



The economical acquisition of nutritive sub- 

 stance in proper amount is a fundamental ne- 

 cessitj' of every organism, and to the condi- 

 tions attendant upon the performance of the 

 nutritive functions must be ascribed the chief 

 causes underlying the development of the plant 

 body. The chlorophyll processes have, there- 

 fore, been the paramount factors in the devel- 

 opment of the shoot, and the absorptive pro- 

 cesses have ruled the differentiation of the root 

 system. 



In an early stage of the existence of the an- 

 cestors of our present plant population they j-j . .. 

 were doubtless thalloid organisms, unicellular existing forms 

 or multicellular, floating in the water, and 

 were perhaps in many instances endowed with 



•Given in an address before the Botanical Club of the Univer- 

 sity of Chicago, Jan. 18, 1897. 



