ECOLOGICAL GROUPING OF PLANTS. 35 
the Leitneria (Z. florédana) in the latter locality and the swamps of 
the estuary of the Apalachicola River present similar striking instances 
of a strange localization, in some cases within an extremely limited 
range. The relation of these woody plants to the flora of a preceding 
geological period, of which the present is the progeny, is clearly 
proved by the remains of almost identical types found buried in the 
-strata of the Middle Tertiary formation of southern Europe and the 
Rocky Mountains. If we regard these plants as the slightly modified 
descendants of types belonging to an ancient flora, which have sur- 
vived the changes our globe has undergone and have found a refuge 
in their present localities, the mystery of their strange isolation finds 
a satisfactory solution. The confinement to a single spot on the 
Warrior of Weviusia alabamensis, with its nearest relation in Japan, 
and of Croton alabamensis to a secluded valley of Little Cahaba River, 
can be accounted for on the same ground. 
PLANT FORMATIONS AND PLANT ASSOCIATIONS. 
The same causes to which the restriction of a plant within a floral 
region is due, give rise to the grouping of species widely differing in 
their natural affinities but equally adapted to accommodate themselves to 
the same conditions. Such assemblages, forming groups of a definite 
character, are by the later writers on plant geography recognized as 
plant formations (Grisebach, Drude); or, with respect to their condi- 
tions of life, their ecological relations as plant associations by Warm- 
ing.* The former authors define a plant formation asa group of plants 
of a definite physiognomic character, such as a forest, a grassy swamp, 
the vegetation of a dune of the seashore. The group may be composed 
either of only one species, as the forests of long-leaf pine or the South- 
ern canebrakes, or characterized by the predominance of species of 
the same family or allied families, as most coniferous forests and our 
forests of oaks and hickories, or composed of an aggregate of species of 
various affinities with some of their features in common, as is the case 
with most forests and prairies. These plant formations impress upon 
a flora its physiognomy, as was long ago noticed by Humboldt. The 
following plant formations, recognized by Engler and Drude,’ will be 
referred to in considering the distribution of plant life in Alabama: 
1. Open plant formations, more or less interrupted, inhabiting— 
a. The strand or beach. 
b. The sands of seashore, hill, and plain. 
¢. Rocks and pebbles. 
1Warming, Oekologische Pflanzengeographie, 1896. 
2Engler und Drude, Die Vegetation der Erde: I, Pflanzenverbreitung auf der Iber- 
ischen Halbinsel, M. Willkomm, p. VII. 1896, 
