36 PLANT LIFE OF ALABAMA. 
2. Compact or continuous plant formations of a uniform type, consist- 
ing— 
d. Of mosses or lichens. 
. Of cespitose grasses (meadows, grassy swales). 
F. Of various herbs (prairies, pastures). 
g. Of gregarious suffrutescent plants or low undershrubs. 
h. Of gregarious larger, woody plants branched from the base 
(thickets). 
:. Of arboreal vegetation (open and dense high forest). 
i. Of paludial plants (riparian swamps, marshes, and bogs). 
1. Of immersed aquatics, floating free or rooted in the soil. 
Considering these natural groups or plant formations in their bio- 
logical aspect and investigating the mutual relationship between the 
plant and the place where it lives (habitat), Warming finds that the 
properties by which a plant is able to adapt itself to the influence of 
_ the various factors to which it is exposed and to hold its own in com- 
petition with its associates are deeply founded in peculiarities of 
anatomical structure as well as in the morphological development of 
its organs and the resulting physiological functions. The presence 
or absence of a type in any given plant formation finds its explanation 
in these peculiar modifications of its organism by which its mode of 
life is regulated, as well as in the ecological relations existing between 
its own life and the life of its associates. 
On these principles Warming has proposed a new classification of 
the plant covering of the globe, recognizing four principal groups 
of associations based on ecological relations, namely: 
(1) Hydrophytic vegetation, forming the associations of plants sur- 
rounded entirely or partially by water or growing in a constantly 
water-soaked soil. 
(2) Xerophytic vegetation, forming the associations of plants con- , 
fined to an arid soil and a dry atmosphere. 
(3) Halophytic vegetation, making up plant associations restricted 
to a saline soil, wet or dry. 
(+) Mesophytic vegetation, including the general vegetation prefer- 
ring a soil and an air of medium humidity. 
Of the various plant associations met with in Alabama, those com- 
posed chiefly of vascular plants will be more thoroughly discussed 
under this classification of Warming. 
It is self-evident that there exist many intermediate forms between 
these groups, and these often render it extremely difficult to assign a 
certain plant association to a place in one or other of the above groups 
or classes. 
Within each of these four principal groups the vegetation is com- 
posed of typical forms of plant life, distinguished as trees, shrubs, 
