No. 17] FLOWERING PLANTS AND FERNS 25 
Plant Succession.—Plant Associations are not fixed but 
are constantly changing, because of the constant changes in 
their environmental conditions. The growth of an assemblage 
of plants, called an Association, adapted to a certain habitat 
and occupying a given area, may be removed by extraneous 
forces and their habitat profoundly changed. In such a ease, 
when the area comes to be reoecupied it will be by an entirely 
different assemblaye. A familiar example is in the clearing 
of new lands for farm cultivation. In our state a rich meso- 
phytic upland forest of mixed broad-leafed trees may be re- 
moved from an area and the soil cultivated until the humus 
and other elements of fertility are reduced. If the area is 
then abandoned to nature it is a well-known fact that the 
same trees, shrubs and hurbaceous vegetation that originally 
occupied it will not immediately repossess it. For the first 
two or three years the growth will be almost wholly weeds, 
none of which, perhaps, occupied the original woodland soil. 
After these, in our state, a scattered growth of persimmon 
sprouts is very liable to share the area with the weeds. Very 
soon, dense patches of seedling loblolly pine spring up, until 
often large areas are thickly covered with a growth of this 
pine. Little undergrowth exists beneath these young pine 
forests, but their needles and twigs, together with decaying 
trunks, gradually add humus to the soil. After the pine 
forests become tall and more light is admitted to the ground, 
young oaks, such as black jack and post oak, begin to invade 
the pines from the edges, and grow into fringing thickets, 
that gradually push into the pine forest. Any break in the 
phalanx of pines is at once occupied by these Xerophytic oaks, 
until we finally find the oaks replacing the pines. Once the 
oaks, with their accompanying thickets of undershrubs, oc- 
cupy a space, the pine seed is no longer able to germinate 
there, and so, eventually the pine forest becomes a mixed 
forest of pine and oak, and finally an oak forest with only an 
occasional pine, the pines being crowded out. The area once 
occupied by the black jack, post oak, and persimmon, the soil 
becomes rapidly enriched, and their occupation is shared by 
all the more mesophytice oaks, elms, hickories and the great 
horde of forms that originally occupied the area. 
