OEIGIN OF DOMESTIC PLANTS. 57 



of growth, grass land is probably a more energetic evaporator and 

 refrigerator than even the forest, but this powerful action is ex- 

 erted, in its full intensity, for a compai-atively 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 cK- 

 matic extremes, or in conservation of geographical surface and 

 outline, as is the soil which nature herseK has planted. 



Origin of Domestio Plcmts. 



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 multipHed tree-fruits of our gardens, as 

 artificially modified and improved forms of wild, self -propagating 

 vegetation. The narratives of botanical travellers have often an- 

 nounced the discovery of the original form and habitat of domes- 

 ticated plants, and scientific journals have described the experi- 

 ments by which the identity of particular wild and cultivated 

 vegetables has been thought to be estabhshed. 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 bread- 

 stuffs and most other esculent vegetables of Europe and the East 

 — ^^are found wild and self-propagatkig in Spanish America, though 

 in forms not recognizable by the common observer as identical 

 with the famihar corn and tuber of modern agriculture. It was 

 lately asserted, upon what seemed very strong evidence, that the 



fanners 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 part due to the fact that, at this period of growth, the roots 

 penetrate down to a permanently humid stratum of soU, 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 Uttle 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 gaias 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 in con- 

 tact with them, is to supply directly elements of vegetable growth. 



