236 



The Living Plant 



In consequence these seed capsules are brought into contact with 

 the cliff, and, moving about more or less, are reasonably sure to 

 push into some crevice where the seeds can be dropped in posi- 

 tion for starting the new plant 

 in its favorite habitat, instead 

 of at the foot of the cliffs. 

 There are two good reasons 

 why I cite this example. In the 

 first place it shows that the 

 phototropism of a part may 

 change — the lever may be 

 thrown — during its own life, 

 though this is not common. 

 In the second place, the seed 

 capsule has obviously no need 

 to get away from the light as 

 such, but simply to get back 

 against the cliff. Since, how- 

 ever, there exists no cliff-ward 

 stimulus, the light, which hap- 

 pens to act in the suitable di- 

 rection, is used for the purpose. 

 Light in this case acts as a 

 foster-stimulus as it were, and 

 may thus be described, in con- 

 trast with the direct stimuli of 

 the examples earlier described. 

 There remains one other class 



Fig. 80. — A cut shoot of Bellflower, kept for r t i j. j^i 11 i 



two days in a chamber lighted wholly ot hght respOUSCS,— the SO-Callcd 



Inrnn?'''^f*;v,^fl^'''''^'*^'^°''*''^'P^°' slccp moveuients. It is very 



totropism of the flowers. ^ "^ 



well known that some leaves 

 droop at night, as in Clovers, Wood-sorrels, Beans, and many 

 other members of the Pea family (figure 82); and most people 

 have seen, at some time or other, the remarkably tight-shut ap- 



