184 



ELEMENTARY STUDIES IN BOTANY 



former showing positive and the latter negative phototropism, 



as explained more fully in the legend. 



By putting together the results of the various tropisms, 



it is evident that an irritable structure (as a growing stem 



or root) responds to several of them at the same time. The 



tap-root, for example, has been 

 shown to respond to the stimulus 

 of gravity, of moisture, and of light, 

 and each response directs it into 

 the soil, so that its direction is de- 

 termined by the sum of all these 

 stimuli. In the same way, the stem 

 is not only positively phototropic, 

 but also negatively geotropic, so 



FIG. 152. The same seedling 

 shown in Fig. 151, completely 

 inverted, and after two days 

 photographed. 



FIG. 153. Seedling of white mustard grown in 

 water and exposed to weak light, showing that 

 the stem is positively phototropic and the root 

 negatively phototropic ; the arrows indicate the 

 direction of the rays of light. 



that the sum of these stimuli determines the direction 

 away from the soil. 



It was stated that all roots are not positively geotropic, 

 for root branches frequently grow horizontally. In the same 



