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 (Linnea) ; 
(3) small ferns and low flowering herbs — pyrola, clintonia, 
trillium ;‘ (4) a zone of tall herbs and low bushes — royal 
fern, cohosh (Actea), 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. Where 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 
