SUN AND SHADE LEAVES 157 



exaggeration of a certain developmental stage. This is well 

 shown in the harebell. In open stations it produces a few 

 rounded leaves and then forms only the linear adult type, 

 these alone appearing on the flowering shoot. By keeping 

 the plant shaded, or by placing an adult plant in the shade, 

 the continued or renewed production of the youth form is 

 forced on the plant. In this case the plant remains 

 immature, for it does not flower. 



The plasticity of such trees as the beech and maple is 

 much more limited. As might be expected, it is not 

 possible to modify the leaves of the seedling so much as 

 those of the adult. Moreover, in the adult, it is easier to 

 force the basal leaves of a shoot to assume shade, or youth, 

 characters, and the apical leaves to assume sun characters, 

 than to reverse these processes. A particular leaf, by 

 virtue of the age of the plant bearing it, or of its position on 

 the shoot, inclines to one or the other type ; it is always 

 easier to exaggerate that type than to reverse it. 



The conditions in which the young plant finds itself 

 are likely to be more or less shaded and moist. Even the 

 harebell sprouting in an open place will be shaded by 

 surrounding grasses, and so also with tree seedlings. Normal 

 development brings the plant gradually to stronger light and 

 drier air. So we have a relation between the environment of 

 the youth form and of the adult shade form, as we have 

 between their structure. 



The influence of intense illumination has been studied 

 by Bonnier (1895). A comparison of plants at high alti- 

 tudes with those of the same species in the plains, showed 

 that the former tend to produce smaller, hairier leaves of an 

 extreme sun type with thick cuticle and numerous stomata. 

 The very high degree of insolation, and particularly the 

 excessive proportion of ultra-violet light, must play an 

 important part in causing these changes, but many other 

 factors are also at work. 



Alpine Plants. — Henrici (1921) has recently investigated 

 the assimilation of alpine plants. The lower temperature 



