336 VEGETABLE PHYSIOLOGY 



necessary for the acquirement of a considerable bulk. 

 Aquatic vascular plants are consequently never very large. 

 The difference between the two groups of aquatic plants 

 spoken of may be well seen in such forms as Cabomba, 

 which bears both submerged and floating leaves. These 

 show respectively the characteristics described in each 



case. 



Some curious adaptations of the organism to its environ- 

 ments are exhibited by certain of these plants which live 

 in marshy surroundings, sometimes being nearly or wholly 

 submerged, and at others, owing to the drying up of the 

 water, growing upon the mud. When the latter fate befalls 

 them, such of their leaves as are adapted to an aquatic life 

 become dried up, and perish. The upper leaves which have 

 always been exposed to the air do not suffer. As growth 

 continues, all the foliage which is produced is of the terres- 

 trial type. On the other hand, when the plant-body is 

 submerged the new leaves are all of the aquatic type. 

 These plants are often spoken of as amphibious. 



Some aquatic plants are saprophytic in their mode of 

 life, flourishing best in water which is contaminated with 

 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 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 xerophytic plants 

 as are woody in habit frequently show considerable ten- 



