Ch. XIV] THE AEROPHYTES 565 



while the conditions of light absorption by chlorenchyma 

 (pages 52-3), explain the further differentiation of the shoot 

 into thin leaves and supporting stem. Again, the usual, and 

 often extreme, scarcity of moisture in the air exposes the 

 aerial parts of plants to desiccation ; and correspondingly 

 such parts are enwrapped with waterproof coverings of 

 epidermis and cork. The waterproof covering, however, 

 is practically impervious to carbon dioxide and oxygen ; 

 and correspondingly we find epidermis and cork pierced 

 by openings, — the stomata and lenticels, through which 

 those gases pass readily. Openings which permit passage 

 of carbon dioxide and oxygen, however, permit also passage 

 of water vapor, and hence loss from the plant (transpiration), 

 with danger of desiccation of the tissues. Correspondingly 

 we find stomata and lenticels guarded by movable cells or 

 special tissue growth, which automatically diminish those 

 openings as the tissues dry out. The method, however, 

 has limitations, and in places where dryness prevails is sup- 

 plemented by reduction in size of the plant, verticality of the 

 tissues, thickness of epidermis, and other xerophytic features 

 (page 69-70), which produce a profound effect upon form and 

 aspect. Again, the rapidity of diffusion of carbon dioxide 

 and oxygen in air permits their effective transfer along 

 slender passages ; and, correspondingly, aerophytes possess 

 systems of continuous intercellular passages, opening through 

 stomata and lenticels, whereby deep-lying tissues are 

 effectively aerated, and massive growth becomes possible. 

 Finally, unlike the water, the air affords practically no 

 buoyant support to plants, which must withstand great and 

 increasing strains as they grow in size. Correspondingly, 

 while plants of low stature and sheltered habitat are her- 

 baceous and soft-bodied, those which grow tall or exposed 

 develop firm woody tissue arranged upon mechanically 

 effective systems. 



These conditions of the land habitats explain the distinc- 

 tive features of aerophytes, — their differentiation into chloro- 



