308 BOTANY PART i 



and causes them to develop as bud-scales. If the foliage leaves are 

 removed in early summer these primordia develop as foliage leaves 

 instead of scale leaves. In a similar fashion the removal of leafy 

 shoots may affect a subterranean rhizome, and cause it to grow out of 

 the soil and form foliage leaves instead of scale leaves. 



Another kind of heterophylly is met with in some plants in 

 which the form of leaves produced during youth differs from those 

 borne on the older plant. It is sometimes possible to bring about 

 a return to the juvenile form when the external conditions under which 

 this arises are again established. Thus in the case of Campanula 

 rotundifolia round leaves can be developed on plants which have 

 formed the subsequent linear leaves by diminishing the intensity of 

 the illumination. In some aquatic plants the submerged leaves belong 

 to the juvenile form, and the floating or aerial leaves to the later 

 adult form. Here also the juvenile form can be induced. This is not 

 always the case, however, for sometimes the growing point has been 

 so profoundly changed that it can only produce the later adult type 

 of foliage. 



The stem also may undergo far-reaching transformations. It may 

 be erect in the case of leafy shoots or creep horizontally on or 

 in the soil ; in twining plants the internodes are greatly lengthened, 

 while they are very short in rosette plants ; there are wide differences 

 in the growth in thickness, in extreme cases the stem becomes a 

 tuber. All these various forms or modes of growth result from 

 definite influences, and can, in part at least, be obtained experimentally 

 even at times and places where they would not occur in the " normal " 

 course of development. 



The formation of tubers in the Potato affords an example of the 

 plasticity of the stem. As is represented in Fig. 203, the tubers 

 usually form at the ends of horizontal stolons which arise from the 

 lower region of the foliage shoot where it is embedded in the soil. 

 The tuber forms by marked growth in thickness of the end of the 

 stolon, and cessation of its growth in length. If, however, the leafy 

 shoot is removed at the proper time, the ends of the stolons grow into 

 erect branches which emerge from the soil and bear foliage leaves. 

 The typical development of the Potato can thus be modified so that 

 no tubers are formed. On the other hand, tubers can be caused to 

 form at other places : for example, at low temperatures the main 

 axis of a particular kind of Potato will remain short, and be trans- 

 formed into a tuber ; in other varieties tubers are produced near the 

 summit of the aerial leafy shoots when the tip of the shoot is 

 darkened. Boussingaultia baselloides is even more plastic than the 

 Potato ; any bud can be induced to form a tuber, and when buds are 

 lacking, internodes or roots may swell into tubers. Apparently the 

 production of a certain amount of reserve material acts as a stimulus 

 leading to the formation of a storage organ. 



