54 



BIOLOGICAL LECTURES. 



few cases at least this connecting link was to be sought in the 

 changes of osmotic pressure determined by the chemical changes 

 which take place in the growing organ. 



But this fact alone does not explain why it is that we get 

 differences in the forms of organs. In order to understand 

 this we must bear in mind that the processes of growth must 

 necessarily be different for different organs, as for example in 

 the formation of a root, and the formation of a stem. As 



FIG. 24. 



growth is a process in which energy is used up in overcoming 

 the resistance to growth, differences of growth can only be 

 determined either by differences in the amount of energy set 

 free in the growing organ or by differences in resistance. Dif- 

 ferences in the energy must be the outcome of differences in the 

 chemical processes which determine growth. Therefore we 

 are led to the idea that differences in the forms of different 

 organs must be determined by differences in their chemical 



