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 calcareous 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 (Silphiwn 

 laciniatum, etc.), species of sunflower (Helianthus), fleabanes (Eri- 

 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 11 are pteridophytes, 

 139 species are monocotyledons, and 77 dicotyledons. 



HYDROCHARIDEAN CLASS. 



Of hydrophytes floating free in water, 9 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, Spirodela), or 



