II 



THE DEVELOPMENT OF IRRITABILITY' 



The economical acquisition of nutritive sub- 

 stance in proper amount is a fundamental ne- 

 cessit^" 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 r^ , ,, 

 were doubtless thalloid organisms, unicellular existing forms 

 or multicellular, floating in the v^ater, 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. 



O 



