288 PRACTICAL COURSE IN BOTANY 
Ill. ZONES OF VEGETATION 
325. The origin of vegetable zones. — The terms “ zone” 
and “ zonation ” are used to express a general tendency of 
plant societies and formations to distribute themselves in 
more or less regular belts or strata, relatively to the varying 
intensity of the prevalent ecological factor of their habitat. 
In almost every locality there exists some special feature — 
a pond, a brook, a small ravine, an isolated hilltop, a deserted 
quarry, a gravel pit, or a railroad cut, — sufficiently distinct 
from the general surroundings to exercise a perceptible 
control over the 
vegetation in its 
immediate vicinity, 
and thus to become 
the starting point 
of a series of plant 
zones that mark the 
decreasing influence 
of the factor con- 
cerned, by their 
change of character 
Fic. 422.—A pioneer colony of sumac growingon a5 they recede from 
a en cutting. (From a photograph by J. M. its point of greatest 
intensity. Starting 
from a barren, exposed hilltop, for example, with a covering 
of dry broom sedge (Andropogon) and fleabane, we encounter 
next an almost desert zone of washed and gullied slopes in 
whose hard, sun-baked soil nothing but a few scrub pines and 
brambles can gain afoothold. This will, perhaps, be succeeded, 
by a straggling belt of sassafras, sumac, and buckthorn, mixed 
with cat brier and blackberry canes, beyond which, at the foot 
of the hill, begins a stretch of meadow, or a bit of woodland 
crossed by a brook, or hollowed into a boggy depression. 
From this new factor originates a second series of zonations, 
passing through all the stages of bog, swamp, shade, and sun 
