336 



VEGETABLE PHYSIOLOGY 



that to which the term Xerophytes has been applied. 

 These inhabit different situations, all of which are charac- 

 terised by presenting to the plant a very small supply of 

 terrestrial water. Many grow in sandy deserts, exposed to 

 great heat, and frequently undergoing long periods of 

 drought. Others grow upon a rocky substratum, and their 

 roots are confined to the crannies and crevices which are 

 present in the rock. Others are found in more temperate 

 countries, occupying light sandy soils which cannot retain 

 any considerable quantity of water. Such xerophy tic plants 

 as are woody in habit frequently show considerable ten- 



FJG. 143. LEAF OF Saxifraga incrustata, SHOWING ABSORBING ORGAN, x 20. 



dency to diminish their leaf-surface, probably to reduce 

 evaporation and conserve their stock of water. They often 

 have many of their branches transformed into thorns or 

 spines, and very frequently their leaves show similar reduc- 

 tion. Others which contain little wood are succulent, and 

 their surfaces are covered by a very thick and tough 

 epidermis, which is strongly cuticularised. Many of those 

 which grow upon rocks have leaves which show special 

 structures for absorbing water from rain or dew. Several 

 species of Saxifrage possess a number of glandular struc- 

 tures upon the teeth of their thick narrow leaves. Each 

 consists of a small mass of cells with delicate walls, which 



