2] UPON THE DIRECTION OF GROWTH 385 



food. Finally, growth must be studied as a response to chem- 

 ical agents which may stimulate the protoplasm to absorb and 

 assimilate them to the degree required by the organism, or 

 which may stimulate the protoplasm to absorb some other sub- 

 stances, as we have seen in the case of zinc sulphate. The 

 quantity and quality of food needed will, moreover, vary with 

 the age and other qualities of the organism. The consumption 

 of food both in quantity and in quality will be closely deter- 

 mined by the demand. All these complexities in the process 

 of nutrition indicate that it, like other processes in organisms, 

 can only be explained on the assumption of a vastly complex 

 molecular organization of the protoplasm. 



2. EFFECT OF CHEMICAL AGENTS UPON THE DIRECTION 

 OF GROWTH CHEMOTROPISM 



One of the most common processes in the early development 

 of organisms is the turning or bending of a filament, tubule, or 

 lamella. The cause of this turning is clearly an unequal 

 growth of the two sides of the organ. When the bending 

 organs are internal, their movements are largely removed from 

 experimental study ; when external, as in plants, they more 

 easily lend themselves to our investigation. The object of 

 this investigation is always to find in how far the direction of 

 these tropic movements is determined or is determinable by 

 external agents. 



In the present section it is proposed to consider in how far 

 tropic movements are determined by chemical agents. At the 

 outset it must be said that the growth which gives rise to these 

 bendings is frequently due to imbibition of water ; and in such 

 cases it may be only temporary. Yet these temporary bendings 

 pass by such insensible gradations into permanent ones that a 

 sharp distinction between the two is impracticable and unim- 

 portant. Rejecting such a classification of the subject, we may 

 adopt one based on the tropic organ. 



1. Chemotropism in the Tentacles of Insectivorous Plants. - 

 This case of chemotropism was the earliest to be observed ; it 

 was DARWIN ('75, p. 76) who first called attention to it. He 

 found that when drops of water or solutions of non-nitrogenous 



