=335. Physical factors.= The physical causes of zonation are by far the most important. They arise from differences in temperature, water, and light. In the large, temperature differences are the most important, producing the great zones of vegetation. In a particular region or habitat, variations of water-content and humidity are controlling, while light, as shown above, is important in the reactions of forest and thicket. Physical factors produce zonation in a habitat or a series of habitats, when there is either a gradual and cumulative, or an abrupt change in their intensity. Gradual, slight changes are typical of single habitats; abrupt, marked changes of a series of habitats. This modification of a decisive factor tends to operate in all directions from the place of greatest intensity, producing a characteristic symmetry of the habitat with reference to the factor concerned. If the area of greatest amount is linear, the shading-out will take place in two directions, and the symmetry will be bilateral, a condition well illustrated by rivers. On the other hand, a central intense area will shade out in all directions, giving rise to radial symmetry, as in ponds, lakes, etc. The essential connection between these is evident where a stream broadens into a lake, or the latter is the source of a stream, where a mountain ridge breaks up into isolated peaks, or where a peninsula or landspit is cut into islands. The line that connects the points of accumulated or abrupt change in the symmetry is a stress line or _ecotone_. Ecotones are well-marked between formations, particularly where the medium changes; they are less distinct within formations. It is obvious that an ecotone separates two different series of zones in the one case, and merely two distinct zones in the other. [Illustration: Fig. 73. Regional zones on a spur of Pike’s Peak (3,800 m.); the forest consists of _Picea engelmannii_ and _Pinus aristata_, the forewold is _Salix pseudolapponum_, and the grassland, alpine meadow (_Carex-Campanula-coryphium_).]