INFLUENCE OF ENVIKONMENT ON PLANTS 343 



sewage or with the products of putrefaction. They are 

 chiefly certain species of Algae or Fungi, but among them 

 may be included a few Mosses and Phanerogams. 



Another class of plants which show a definite response 

 in their structure to the conditions in which they live is 

 that to which the term XeropJiytes has been applied. These 

 inhabit different situations, all of which are characterised 

 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 



FIG. 143. LEAF OF Saxifraga incrustata, SHOWING ABSORBING ORGAN. X 20. 



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 con- 

 siderable quantity of water. Such xerophytic plants as 

 are woody in habit frequently show considerable tendency 

 to diminish their leaf-surface, probably to reduce evapora- 

 tion 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 reduction. 

 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 



