898. The large boat-shaped, tapering pods, in late autumn, are packed with oval, flattened, brownish seeds, which overlap each other in rows like shingles on a roof. These make a pretty picture as the pod in drying splits along the suture on the convex side, and exposes them to view. The silky tufts of numerous long, delicate white hairs on the inner end of each seed, in drying, bristle out, and thus lift the seeds out of their enclosure, where they are caught by the breeze and borne away often to a great distance, where they will germinate if conditions become favorable, and take their places as contestants in the battle for existence.
899. The virgin’s bower.—The virgin’s bower (Clematis virginiana), too, clambering over fence and shrub, makes a show of having transformed its exquisite white flower clusters into grayish-white tufts, which scatter in the autumn gusts into hundreds of arrow-headed, spiral plumes. The achenes have plumose styles, and the spiral form of the plume gives a curious twist to the falling seed ([fig. 485]).
Fig. 485.
Seed distribution of virgin’s bower (clematis).
[CHAPTER XLVI.]
VEGETATION IN RELATION TO ENVIRONMENT.[47]
I. Factors Influencing Vegetation Types.
900. All plants are subject to the influence of environment from the time the seed begins to germinate until the seed is formed again, or until the plant ceases to live. A suitable amount of warmth and moisture is necessary that the seed may germinate. Moisture may be present, but if it is too cold, germination will not take place. So in all the processes of life there are several conditions of the environment, or the “outside” of plants, which must be favorable for successful growth and reproduction. Not only is this true, but the surroundings of plants to a large extent determine the kind of plants which can grow in particular localities. It is also evident that the reaction of environment on plants has in a large measure caused them to take on certain forms and structures which fit them better to exist under local conditions. In other cases where plants have varied by mutation ([p. 338]) some of the new forms may be more suited to the conditions of environment than others and they are more apt to survive. These conditions of environment acting on the plant are factors which have an important determining influence on the existence, habitat, habit, and form of the plant. These factors are sometimes spoken of as ecological factors, and the study of plants in this relation is sometimes spoken of as ecology,[48] which means a study of plants in their home or a study of the household relations of plants. These factors are of three sorts: 1st, physical factors; 2d, climatic factors; 3d, biotic factors.
901. Physical factors.—Some of these factors are water, light, heat, wind, chemical or physical condition of the soil, etc. Water is a very important factor for all plants. Even those growing on land contain a large percentage of water, which we have seen is rapidly lost by transpiration, and unless water is available for root absorption the plant soon suffers, and aquatic plants are injured very quickly by drying when taken from the water. Excess of soil water is injurious to some plants. Light is important in photosynthesis, in determining direction of growth as well as in determining the formation of suitable leaves in most plants, and has an influence in the structure of the leaf according as the light may be strong, weak, etc. Heat has great influence on plant growth and on the distribution of plants. The growth period for most vegetation begins at 6° C. (= 43° F.), or in the tropics at 10°-12° C., but a much higher temperature is usually necessary for reproduction. Some arctic algæ, however, fruit at 1.8° C. The upper limit favorable for plants in general is 45°-50° C., while the optimum temperature is below this. Very high temperatures are injurious, and fatal to most plants, but some algæ grow in hot springs where the temperature reaches 80°-90° C. Some desert plants are able to endure a temperature of 70° C., while some flowering plants of other regions are killed at 45° C. Some plants are specifically susceptible to cold, but most plants which are injured by freezing suffer because the freezing is a drying process of the protoplasm ([see p. 374]). Wind may serve useful purposes in pollination and in aeration, but severe winds injure plants by causing too rapid transpiration, by felling trees, by breaking plant parts, by deforming trees and shrubs, and by mechanical injuries from “sand-blast.” Ground covers protect plants in several ways. Snow during the winter checks radiation of heat from the ground so that it does not freeze to so great a depth, and this is very important for many trees and shrubs. It also prevents alternate freezing and thawing of the ground, which “heaves” some plants from the soil. Leaves and other plant remains mulch the soil and check evaporation of water. The influence of the chemical condition of the soil is very marked in alkaline areas where the concentration of salt in the soil permits a very limited range of species. So the physical and mechanical conditions of the soil influence plants because the moisture content of the ground is so closely dependent on its physical condition. Rocky and gravelly soil, other things being equal, is dry. Clay is more retentive of moisture than sand, and moisture also varies according to the per cent of humus mixed with it, the humus increasing the percentage of moisture retained.