Fig. 46. Series for producing hydrophytic forms under control: 1, amphibious; 2, floating; 3, competition; 4, submerged.
Reciprocal transfers may be made by means of seed or plant. Since the experiment is a complex one, all the care possible should be taken to make sure that the plants become established in the reciprocal situations, and consequently, it is often advisable to transfer both seeds and plants. Reciprocal transfer is of paramount value in solving the problem which bog plants present. A slight modification of the method makes it possible to obtain experimental evidence of the polyphyletic origin of species in consequence of adaptation. In an experiment mentioned elsewhere, the transfer of Kuhnistera purpurea to the area occupied by K. candida, and vice versa, is designed to show whether one has been derived from the other. If the two species are moved into an area which contains more water than that usually occupied by K. purpurea, and less water than is found where K. candida habitually grows, the resulting modifications will throw much light upon the origin of polyphyletic species. In this connection, it hardly needs to be pointed out that this simple transfer of a species to several separated areas of a new habitat may often furnish complete proof that a new form may arise at different times, and at different places.
Fig. 47. Control ecad of Ranunculus sceleratus, holard 10% (50 cc.).
194. Modification of the habitat. Efficient changes in the habitat are brought about by increasing or decreasing the water-content, or by varying the light intensity between sunshine and the diffuse light of deep forests. Humidity can not well be regulated except in so far as it is connected with water-content. Since its effects merge with those of the latter, its modification is unnecessary. An increase in water-content is readily brought about by irrigation. A stream may be dammed and its water allowed to spread over the area to be studied, or the water may be carried to the proper place by deflecting the stream or by digging a canal. The construction of earth reservoirs makes it possible to obtain almost any per cent of soil water by varying the size of the reservoir or the height of the wall or bank. Near a base station, such as Minnehaha, where there is a simple system of water-works, the experimental area may be watered whenever desirable by means of a hose. Water-content may be readily decreased by drainage, or by the deflection of a stream. When such means are not available, as in the case of extensive marshes, hummocks may be used or constructed, and the soil blocks containing plants placed upon them. By the use of sand or gravel, the water-content of mesophytic areas can be reduced in a similar manner, or by surrounding the plant in situ with either of these soils which hold little water. In meadows, especially, the addition of a large quantity of alkaline salts decreases the amount of available water, while the holard may be reduced by denuding the soil about the plants concerned.
Fig. 48. Control ecad of Ranunculus sceleratus, holard 40% (200 cc.).
In sunny habitats, the light intensity is most easily reduced by means of cloth awnings, which can be put in place conveniently. It is not a difficult matter to produce effective shade by using shrubs or small trees for this purpose. This plan is especially advantageous in habitats too remote to make frequent visits feasible. When a shrub or tree is used, the experiment necessarily requires a longer time, though this disadvantage is partly compensated by the fact that the shelter requires practically no attention after the shrub is once established. Forest plantations furnish excellent examples of this kind of experiment. On the other hand, clearings afford the only examples of habitats modified in such manner as to increase the light. In nature, the diffuse light in which shade plants grow is due to the presence of tall plants, chiefly shrubs and trees, and an increase in the light intensity is possible only through the thinning-out or removal of the plant screen. This is a task of considerable magnitude in forests, but it can be readily accomplished in thickets and at the edges of woodlands. It is quite practicable to establish a series of awnings or clearings of various light values, but the labor required is hardly worth while when it is recalled that the method of transfer makes it possible to take advantage of the various intensities already found in nature.