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, 

 j/ however, may respond to these 



FIG. 88. -A seedling of stimuli in a very different way, usu- 

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



water and exposed to j. i v i i 



weak light, showing the zontal direction, and being mainly 

 positive phototropism transversely qeotr.omc. The leaves. 



of the stem and the if 



negative phototropism also, are usually neither positively 

 ISteThedtcZ'of nor negatively phototropic, but are 

 the rays of light. directed horizontally, being trans- 



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



