SKCT. n 



PHYSIOLOGY 



197 



checking transpiration during a temporary insufficiency of the water-supply. In 



Fig. lS-2.—rM0idia mammiUaria from New Zealand, showing the cushiou-like shape of the 

 individual plant. (From Schimper's Plant-Geography.) 



districts subject to droughts of weeks' or months' duration, such as deserts, rocky 



country, and other regions with little rainfall, 



only such plants can flourish as are able either to 



withstand a complete drying up without injury 



(p. 190), 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, this 



may be retained for later use. 



Protection against excessive transpiration is 

 afforded by the formation of cork or cuticular 

 coverings and in exceptional cases coverings of 

 resin, by the reduction in the number and size 

 of the stomata, their occurrence in cavities or 

 depressions, and the more or less complete stop- 

 ping of the opening by waxy substances. The 

 rolling up of the leaves, the stomatiferous surface 

 being on the concave side, as well as the develop- 

 ment 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 tran- 

 spiration is undoubtedly obtained by the reduction 

 of the transpiring surfaces, either through a 

 diminution 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. 182). 



The upright position of the leaves, or the 

 substitution of expanded, perpendicularly directed 

 leaf-stalks for the leaves (Phyllodes), particularly 

 cliaracterises the flora of Australia (Fig. 183). A 

 clothing of hair, on the other hand, protects the leaves of some alpine plants 





Fi(i. 1S3. — Aaacia marginata, with 

 vertically placed phyllodes. (Fiom 

 Schimper's Plant-Geography.) 



