PLANTS OF XERIC HABITATS 171 



Annuals, which live through the critical season only as seeds, rep- 

 resent the extreme of temporary reduction of transpiration surface. 

 Dwarf ness of habit is a ])rominent xeric feature and the resulting 

 compactness in arrangement of branches and leaves, as well as the 

 closeness to the ground, afford considerable protection. 



The more minute structural features of xerophytes are no less 

 significant than are the more obvious characteristics. Commonly 

 the epidermis is thick and highly cutinized, and often it is super- 

 ficially coated with wax, resin, or varnish. In woody stems there is a 

 prominent bark development, the cork layers, in particular, being of 

 great significance in checking transpiration. The leaf and stem 

 surfaces frequently are covered with hairs, and spinescence is also 

 common. Spinescence is of no significance in protection from tran- 

 spiration but in some plants the spines probably do afford protection 

 from herbivorous animals. Stomata occur only on protected sur- 

 faces, chiefly the undersides of leaves, and often are placed at the 

 bases of pits or other depressions and may be further protected by 

 hairs or heavy cutinization or both. 



jNIany xerophytes are succulent (Fig. 77); many contain latex; 

 and oils and resins often are abundantly developed. The osmotic 

 pressure of the cell sap usually is very high, especially in woody 

 plants and in plants of alkaline soils and in most xerophytes there is 

 a marked ability to endure wilting without suffering death. Con- 

 ducting tissues are well developed, as is lignification, and annual 

 rings are prominent. Bast fibers and other mechanical elements 

 reach their highest development in xerophytes. Among the most 

 severe of xeric habitats are the alkali plains and basins where exces- 

 sive climatic aridity is supplemented by a soil in which highly con- 

 centrated salts make absorption extremely difficult. Succulents 

 with sap of high osmotic pressure seem to be best fitted for existence 

 under such conditions but there are some places where the alkalinity 

 is so great that plant life is practically excluded. 



Some xerophytes, especially rock lichens, are entirely lacking in 

 •the ordinary xeric structures but are able to withstand prolonged 

 desiccation and then quickly revive with the coming of rains. Aside 

 from lichens and mosses absorption through aerial organs is relatively 

 rare in xerophytes. Tubers, bulbs, and corms are especially char- 

 acteristic of xeric habitats and are of great advantage because of 

 their ability to develop rapidly at the beginning of a rainy season. 

 Vegetative reproduction, however, is not prominent and the produc- 

 tion of flowers and fruits is favored by xeric conditions. 



