RESPONSE OF THE PLANT TO ITS SURROUNDINGS 279 
Hydrophytes, or water plants, those that require 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 | Fis. 415.—A water — 
plant (Limnophila), 
generally be found to have no root system, with water leaves and 
or very small ones, because they absorb 3H leaves and transi- 
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 
