04 A TEXT-BOOK OF BOTANY 
interval, known as reaction time, which is generally much 
longer in plants than in animals. The reaction time may 
Fic. 88.—A seedling of 
white mustard grown in 
water and exposed to 
weak light, showing the 
positive phototropism 
of the stem and the 
negative phototropism 
of the root; the arrows 
indicate the direction of 
the rays of light. 
be several hours, but the movement 
of the leaves of the sensitive-plant 
(§ 17) and the snapping shut of the 
leaves of Dionewa (§ 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, 
however, may respond to these 
stimuli in a very different way, usu- 
ally extending in a more or less hori- 
zontal direction, and being mainly 
transversely geotropic. The leaves, 
also, are usually neither positively 
nor negatively phototropic, but are 
directed horizontally, being trans- 
Fic. 89.—A series in the germination of the scarlet runner bean. 
