94 



A TEXT-BOOK OF BOTANY 



interval, known as reaction time, which is generally much 

 longer in plants than in animals. The reaction time may 

 be several hours, but the movement 

 of the leaves of the sensitive-plant 

 (17) and the snapping shut of the 

 leaves of Dioncea ( 20) follow the 

 stimulation with remarkable prompt- 

 ness. 



The main stem in most cases is 

 positively phototropic, as shown before 

 (Figs. 86 and 87); but it is also 

 negatively geotropic. The branches, 

 jf - however, may respond to these 



FIG. ss.-A seedling of stimuli in a veI T different way, usu- 

 white mustard grown in ally extending in a more or less hori- 



water and exposed to .L i j- j.' 11- i 



weak light, showing the zontal direction, and being mainly 



positive phototropism transversely aeotromc. 



of the stem and the 



The leaves, 

 negative" phototropism also, are usually neither positively 



of the root; the arrows nor ne g a tively phototropic, but are 

 indicate the direction of J XT 



the rays of light. directed horizontally, being trans- 



FIG. 89. A series in the germination of the scarlet runner bean. 



