INTRODUCTION xi 
advanced, or drove the more distant plants southward. When the ice 
receded to the north some plants at the south doubtless succumbed as the 
temperature rose to a point beyond their enduranee more rapidly than 
they could migrate; while others accommodated themselves to the new 
conditions, and traveled back to their former, more northern latitudes. 
Thus our present plant population shows the results of invasions from 
the north. On the other hand, peninsular Florida, and in a lesser degree 
the more northern Gulf Coast, received many elements from the West 
Indies near at hand, and from northern South America, carried over a 
narrow ocean area by birds, winds, and currents. The physiographic struc- 
ture. of the area involved furnishes a sliding seale, either from the boreal 
to the tropical or vice versa, so that any plant which gains a foothold in 
the area might find conditions suitable for its growth and maintenance. 
The above remarks apply mainly to the eastern part of the area; here we 
find a very dense plant population, specifically and generically considered. 
In the western part there are less differences in elevation, and much simpler 
geologic history; here there is sparser plant-population both in density and 
variety. 
There are many locally peculiar minor plant areas. For example, 
capes, peninsulas, coastal sand-dunes, river-deltas, river-valleys or water- 
sheds, swamps, marshes, plateaus, mountain-valleys, mountain ranges, and 
mountain peaks, each harboring peculiar or endemic plants. 
he accompanying map shows the territory included in the South- 
eastern States and the Southcentral States. The Mississippi River divides 
the area into approximate halves. Both areas are shown for convenience 
in tracing the geographic distribution of the plants, and to show the rela- 
tion of the floras of the two areas. These are closely related at the boun- 
dary line, for they both naturally merge in the ancient delta of the Mis- 
sissippi River or the present Coastal Plain part of the Mississippi water- 
shed. 
ir the HL of each species, and separated by a dash 
from it (or from synonyms or common names whieh may be introduced at 
this Pen i habitat, Lie province, and the geographic range are stated. 
tat—Not only are the physical features briefly characterized, but 
in eases w UR it seems noteworthy, something is stated as to the chemical 
nature of the soil, that is whether it is acid, neutral, or alkaline, whether 
silieeous or ealeareous. When the habitat is given as pinelands, the soil 
may be inferred to be acid, except in southern Florida, where the pine- 
woods cover limestone rocks and have more or less alkaline soil. Other- 
wise, where no statement is made as to soil reaetion, either no data are 
available or else the plant appears to be indifferent to this factor. 
Provinces.—Each species is here assigned to one or more physiographie 
provinces. These are used instead of life-zones because the latter do not 
