RESPONSE OF THE PLANT TO ITS SURROUNDINGS 289 



plants, back to the prevailing type of the region. Moisture 

 is really the controlling factor in both cases, its influence 

 in the first being negative, — that is, inversely, — and in the 

 other, positive, or directly proportioned to the quantity 

 present. 



326. Direction of zonation. — When the direction in which 

 the controlling factor changes is horizontal, as with soil and 

 water, the zonation will be horizontal; when, as in the case 

 of light, it is vertical, the zonation or stratification will be 

 vertical. Examples of this can be observed in the growth of 

 almost any forest area, the natural order of succession being : 

 (1) a ground layer of mosses and fungi ; (2) low, creeping 

 vines, — partridge berry, trailing arbutus, twinflower (Linncea) ; 

 (3) small ferns and low flowering herbs — pyrola, clintonia, 

 trillium ; (4) a zone of tall herbs and low bushes — royal 

 fern, cohosh {Actoea), blueberries; (5) tall herbs and shrubs, 

 small trees, and climbing vines — kalmia, dogwood, farkle- 

 berry, smilax, Virginia creeper ; (6) tall treetops towering up 

 into full sunlight. 



When the physical cause of intensity is a central area, such 

 as a pond or a hilltop, the zonation will be concentric ; that is, 

 the different belts will succeed each other in widening circles 

 more or less complete. ^Vhere the controlling cause extends 

 in a line, as a river, or a chain of mountains, the zones run in 

 parallel belts on each side of it, and the zonation is bilateral. 

 In any case, however, it is seldom regular, being frequently 

 broken and interrupted through the intervention of other 

 factors. Nor must precisely the same kind of plants be 

 always looked for in similar situations, though their place is 

 usually occupied by kindred species and genera. The com- 

 mon pitch pine, for instance, of the Northern sand barrens 

 is represented in sandy districts farther south by the tall, 

 long-leaved pine, a kindred species. 



327. Succession. — Zonation is a regular succession of 

 different kinds of plants in space ; there is also an analogous 

 succession in time, as, when the vegetation of a locality is 



