212 



BOTANY 



I'AIIT I 



FIG. 106. Ve'muca. <v>jj;, .-. iVr.v, with the habit 

 of a Cypress, from New Zealand. '(From 

 SCHIMPER'S Plant-Geography.) 



liquid even in the dry air and under the arid conditions natural to 

 the plants. The superfluous water is discharged by a few plants, the 



Pumpkin, for example, into the 

 cavities of their stems and leaf-stalks, 

 and is again absorbed from these 

 reservoirs when needed ( 33 ). 



Special Contrivances for regulating 

 the Water-supply. Almost all the higher 

 plants possess, in the power to close their 

 stomata, a special means of checking 

 transpiration during a temporary insuflici- 

 ency of the water-supply. In districts 

 subject to droughts of weeks' or months' 

 duration, only such plants can flourish as 

 are able either to withstand a complete 

 drying up without injury (p. 195), or to 

 exist for a long time on a scanty supply of 

 water (xerophytes). This last case is only 

 rendered possible by the extreme reduction 

 of transpiration, or by the formation of 

 organs in which, in times of a superfluity 

 of water, it may be retained for later use. 

 Such protection against excessive 

 transpiration is afforded by the formation 



of cork or cuticular coverings and in exceptional cases coverings of resin (Fig. 192), 



by the reduction in the number and size of the stomata, their occurrence in cavities 



or depressions, and the more or less complete 



filling of the opening by waxy substances. 



The rolling up of the leaves, the stomatiferous 



surface being on the concave side, as well as 



the development of thick growths of hair, or of 



a covering of star-shaped or scaly hairs, and the 



assumption of a vertical position to avoid 



the full rays of the sun, are also measures 



frequently adopted to lessen transpiration. 



The most efficient protection, however, from 



too great a loss of water by transpiration is 



undoubtedly obtained by the reduction of the 



transpiring surfaces, either through a diminu- 

 tion in the size of the leaves or through their 



complete disappearance. The same result may 



be obtained by the crowding of the branches 



of the plant to form a dense cushion (Fig. 



193). 



The upright position of the leaves, or the 



substitution of expanded, perpendicularly 



directed leaf-stalks for the leaves (Pnvi. LOOKS), particularly characterises the 



flora of Australia (Fig. 194). A clothing of hair, on the other hand, protects 



the leaves of some South African Proteaceae (e.g. Lcucadeiulron argenteum). 



Some of the Gramineae (Stipa capillata, Festuca alpestris, Sesleria lenuifolia, 



dured leaves may be 

 globose shoots. 



in the upper 



