18 MISSISSIPPI STATE GEOLOGICAL SURVEY (Bull. 
constitute what are called plant Associations, Societies, and 
I‘ormations. 
Ecological Factors—These are the factors which operate 
—gome one usually predominating—to bring plants together 
into groups living under similar conditions. Some of the most 
important of these factors will be briefly noticed. 
Water—The water relation of plants varies from complete 
submergence in some aquatic forms, through medium eondi- 
tions of soil moisture suitable for the bulk of ordinary vegeta- 
tion, to extreme desert conditions where the dearth of water 
precludes the existence of any except a scattered growth of 
specially organized land plants. According to the water rela- 
tion of plants, three great Vegetation Types are recognized: 
Hydrophytes, Xerophytes, and Mesophytes. 
Hydrophyies.—Plants of this group live under conditions 
of abundance of water. Water is undoubtedly one of the 
most important ecological factors, since plants in the active 
condition will die within a very short time if entirely deprived 
cf it. To many plants, however, too much water is almost as 
detrimental as none at all. Even among hydrophytes the 
amount of exposure to water varies greatly. As already 
stated, some groups are completely submerged in water, and all 
their life processes take place under water, except that in a few 
cases pollination of the flowers takes place upon the surface 
of the water. Submerged aquatics are usually specially organ- 
ized to suit the conditions under which they live. Roots may 
be entirely absent, as in free-floating algae; or may serve 
merely as holdfasts, for attachment, as in pond-weeds. The 
leaves are usually pale, and are either narrow and grass-like, 
or finely divided into thread-like divisions. The stems show 
little supporting and conducting tissue, but air cells are numer- 
ous, buoying up the stem in its watery medium. In all these 
modifications in which these aquaties differ from ordinary 
lend plants we see exemplified the economy of nature, in the 
absence of organs not needed in a submerged aquatic but ne- 
cessary in a plant growing under ordinary conditions on land. 
Other groups of hydrophytes are partly submerged in 
