RESPONSE OF THE PLANT TO ITS SURROUNDINGS 279 



Hydrophytes, or water plants, those that requke abundant 

 moisture. 



Xerophytes, or drought plants, those that have adapted 

 themselves to desert or arid conditions. 



Mesophytes, plants that live in conditions intermediate 

 between excessive drought and excessive 

 moisture. To this class belong most of 

 our ordinary cultivated plants and the 

 greater part of the vegetation of the globe. 



Halophytes, " salt plants," is a term 

 used to designate a fourth class, based not 

 directly upon the water factor, but upon 

 the presence of a particular mineral in the 

 water or the soil which they can tolerate. 

 They seem to bear a sort of double rela- 

 tion to hydrophytes on the one hand and 

 to zerophytes on the other. 



318. Hydrophyte societies. — These em- 

 brace a number of forms, from those in- 

 habiting swamps and wet moors, to the 

 submerged vegetation of lakes and rivers. 

 An examination of almost any kind of 

 water plant will show some of the physio- 

 logical effects of unlimited moisture. Take 

 a piece of pondweed, or other immersed 

 plant, out of the water and notice how com- 

 pletely it collapses. This is because, being 

 buoyed up by the water, it has no need to 

 spend its energies in developing woody 

 tissue. Floating and swimming plants will 

 generally be found to have no root system 

 or very small ones, because they 

 their nourishment through all parts of the 

 epidermis directly from the medium in which they live. 

 That they may absorb readily, the tissues are apt to be soft 

 and succulent and the walls of the cells composing them 



Fig. 415. — A water 

 plant {L im nophila) , 

 with water leaves and 



absorb f?"" 'fr' """"^ ^'"'''^'' 



tional forms. 



