INFLUENCE OF ENVIRONMENT ON PLANTS 348 
sewage or with the products of putrefaction. They are 
chiefly certain species of Alge 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 Xerophytes 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 
Fra. 143.—Lear or Saxifraga inerustata, sHowiIne 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 
