PLANT ASSOCIATIONS 945 



protective mantle of snow prevents the loss of water by transpiration in 

 the seasons during which absorption is impossible. The leaves of alpine 

 and arctic xerophytes are largely sclerophyllous, hairy leaves being rather 

 less abundant than in deserts. The perennial habit is almost universal, 

 the shortness of the season scarcely permitting annuals to complete their 

 life cycle. Anthocyans are prominently developed, resulting in showy 

 leaves and flowers. Palisade tissue is more pronounced in alpine than 

 in arctic xerophytes. 



In climates where the rainfall and the temperature are such as to facili- 

 tate their development, there are many habitats where the local condi- 

 tions prevent for a time the appearance of mesophytes. Among such 

 habitats rocky and sandy areas are of first importance. In rocky regions 

 the pioneer plants often are lichens, which are able to grow on the bare 

 rock surface ; with these come many crevice plants. By the accumula- 

 tion of humus, the growth of other plants is made possible, and after a 

 time a thicket develops, and later a forest, in which pines and junipers 

 may play an important part. In sandy regions the instability of the 

 soil usually inhibits the development of a pioneer lichen stage, whereas 

 xerophytic herbs and shrubs make their appearance sooner than on 

 rocks. The subsequent stages on rock and sand are much the same. 



Mesophytic associations. Mesophytes in their structural charac- 

 teristics are in many respects intermediate between hydrophytes and 

 xerophytes. There is a prominent development of progeotropic roots 

 with abundant root hairs. The foliage reaches a maximum develop- 

 ment, and the leaves are relatively large and thin, while the thin trans- 

 parent epidermis and the abundant chlorophyll together cause the leaves 

 to appear dark green. Stomata usually occur on both leaf surfaces, ex- 

 cept in trees, and the guard cells possess a maximum capacity for move- 

 ment. Cutinization is moderate, except in such evergreens as the hem- 

 lock and the India-rubber tree. The under epidermis commonly has 

 wavy lateral walls, contrasting thus with the straight walls in hydrophytes 

 and xerophytes. 



The most representative mesophytic associations are, on the one hand, 

 various forests, and, on the other, certain grasslands. The mesophytic 

 forests as a class are the culminating vegetation of any region where they 

 grow, since they represent not only the most luxuriant kind of plant 

 association, but also because they form the terminal member of the suc- 

 cessional series in such climates as are humid enough to support them over 

 extensive areas. It is in the mesophytic forests that humus accumula- 



