The Origin of Species 343 



provided with stipules, insignificant leaf-like ap- 

 pendages of the leaf-base. If the blade of leaf is 

 removed, these stipules will often become very much 

 enlarged and take over the duties of the destroyed 

 leaf blade. A similar change into a flat blade has 

 been observed in the slender tendrils which termi- 

 nate the leaf in certain plants, like the pea, when the 

 leaflets have been removed. In some species of 

 ferns, the spore-bearing leaves are much smaller 

 than the large sterile leaves, and normally develop 

 comparatively little green tissue. If, however, the 

 sterile leaves, which are the principal photosynthetic 

 organs, are removed while young, the sporophylls 

 which arise later will assume more or less perfectly 

 the character of the amputated fronds. By cutting 

 back the leafy stem of a potato plant, the under- 

 ground shoots which normally develop into the tu- 

 bers, will appear above the ground and develop into 

 leafy shoots, while if the young tubers are constantly 

 removed, the reserve food which would normally 

 be stored up in these will accumulate in tubers 

 formed above the ground from some of the aerial 

 shoots. A familiar case of substitution is that oc- 

 curring in many conifers. If the leading shoot of 

 a pine or fir is destroyed, a lateral shoot below it 

 will usually grow upward and take its place. This 

 accelerated growth and change of position are sup- 

 posed to be due to the diverting of the flow of 

 nutritive matter from the destroyed apical shoots, 

 to one of the lateral ones. 



