The Earth as Modified by Human Action eBook

prevent the evaporation of moisture from its surface by wind and sun.

[Footnote:  It is impossible to say how far the abstraction of water from the earth by broad-leaved field and garden plants—­such as maize, the gourd family, the cabbage, &c.—­is compensated by the condensation of dew, which sometimes pours from them in a stream, by the exhalation of aqueous vapor from their leaves, which is directly absorbed by the ground, and by the shelter they afford the soil from sun and wind, thus preventing evaporation.  American farmers often say that after the leaves of Indian corn are large enough to “shade the ground,” there is little danger that the plants will suffer from drought; but it is probable that the comparative security of the fields from this evil is in port due to the fact that, at thin period of growth, the roots penetrate down to a permanently humid stratum of soil, and draw from it the moisture they require.  Stirring the ground between the rows of maize with a light harrow or cultivator, in very dry seasons, is often recommended as a preventive of injury by drought.  It would seem, indeed, that loosening and turning over the surface earth might aggravate the evil by promoting the evaporation of the little remaining moisture; but the practice is founded partly on the belief that the hygroscopicity of the soil is increased by it to such a degree that it gains more by absorption than it loses by evaporation, and partly on the doctrine that to admit air to the rootlets, or at least to the earth near them, is to supply directly elements of vegetable growth.] At a certain stage of growth, grass land is probably a more energetic evaporator and refrigerator than even the forest, but this powerful action is exerted, in its full intensity, for a comparatively short time only, while trees continue such functions, with unabated vigor, for many months in succession.  Upon the whole, it seems quite certain, that no cultivated ground is as efficient in tempering climatic extremes, or in conservation of geographical surface and outline, as is the soil which nature herself has planted.

ORIGIN OF DOMESTIC PLANTS.

One of the most important questions connected with our subject is:  how far we are to regard our cereal grains, our esculent bulbs and roots, and the multiplied tree fruits of our gardens, as artificially modified and improved forms of wild, self-propagating vegetation.  The narratives of botanical travellers have often announced the discovery of the original form and habitat of domesticated plants, and scientific journals have described the experiments by which the identity of particular wild and cultivated vegetables has been thought to be established.  It is confidently affirmed that maize and the potato—­which we must suppose to have been first cultivated at a much later period than the breadstuffs and most other esculent vegetables of Europe and the East—­are found wild and self-propagating in Spanish America,