288 



PRACTICAL COURSE IN BOTANY 



m. 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 

 as they recede from 

 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 a foothold. 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 



Fig. 422. — A pioneer colony of sumac growing on 

 a railroad cutting. {From a photograph by J. M. 

 Coulter.) 



