48 PLANT LIFE OF ALABAMA. 
OPEN-LAND OR CAMPESTRIAN FLORA (XEROPHILE PLANT ASSOCIATIONS. ) 
Under a climate so highly favorable to tree growth and with soil 
conditions which in general present no obstacles to the development 
of an arboreal vegetation, there is in Alabama, as in the adjoining 
States and the corresponding regions of the Southern Atlantic States, 
but little room left for the characteristic vegetation of open-plain or 
treeless mountain and hill country, especially if we exclude the vege- 
tation of herbs and undershrubs of the open rolling pine barrens, of 
the pine flats of the coast with their scattered tree growth, and of the 
scantily shaded rocky crests of the mountains. It is only on the com- 
paratively small tracts of the Cretaceous plain where the underlying 
limestone strata come near the surface and the covering of the rich 
black caleareous soil becomes too shallow to permit the roots of the 
trees to gain a firm hold, that arboreal vegetation is completely 
excluded. In these localities—the so-called bald prairies, low knolls, 
or broad swells of limited extent, with the soil not deep enough to be 
plowed—many of the typical plants of the eastern North American 
prairie have found a refuge, from which they have spread over the 
borders of fields, open waysides, pasture and waste grounds, and worn- 
out and abandoned lands. In such localities the original types have 
to contend for the possession of the ground with many introduced 
and adventive weeds, the hardy offspring of species originating in the 
exposed plain. Most of the native typical plants of these remnants 
of the prairies, and of the open in general, are also common to the 
prairies of the Mississippi Valley from the Wabash to the valley of 
the Colorado River in Texas. Most of the rosinweeds (S7/phéuim 
laciniatum, ete.), species of sunflower (//e/ianthus), fleabanes (/7r7- 
geron), Rudbeckias, and other tall, coarse composites are character- 
istic of the prairie flora; most of the species of the pea family and 
most of the umbellifers and grasses inhabiting the prairies, open 
borders of fields, and pasture grounds in the central and northern 
part of the State, have also their home on the prairies of Illinois, 
Missouri, southern Arkansas, and eastern Texas, 
WATER AND SWAMP FLORA (HYDROPHYTIC PLANT ASSOCIATIONS). 
Plants of these associations are most prominently represented in the 
lower pine region of the Coast plain. Among the 227 species of vas- 
cular hydrophytes so far observed in Alabama I1 are pteridophytes, 
139 species are monocotyledons, and 77 dicotvledons. 
HYDROCHARIDEAN CLASS. 
Of hydrophytes floating free in water, # species are known in the 
State. They are kept afloat by their thallus or thallus-like stems, as 
in Azolla and duckweeds (Lemna minor, L. trisulcata, Spirodeda), or 
