58 ORIGIN OF DOMESTIC PLANTS. 
evaporation of moisture from its surface by wind and sun.* 
Ata 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 com- 
paratively 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 Planis. 
One of the most important questions connected with our sub- 
ject 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-propa- 
gating vegetation. The narratives of botanical travellers have 
often announced the discovery of the original form and habitat 
* Tt 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 evilis in part due to the fact that, at this 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. 
