15 



time to new genera. To this point I have paid particular attention in Virginia, and find that how- 

 ever the species may vary in different soils, there is a sequence of natural families sufficiently 

 apparent. Where the land is remarkably rich, the plants first developed are species of the families 

 ChenopodiaceEe, Solanacese, Polygonaceai — these give place to Malvaceae, Compositae, and Umbelli- 

 ferse; and finally species of Leguminosas, Rosacese, and Graminese succeed. It is not asserted that 

 other families are absent, but these are so fully developed as to be characteristic of the vegetation. 

 This natural succession differs with the latitude, soil, and degree of moisture ; but whatever may be 

 the families, it is sufficiently apparent that the i:)lants of new soils, or rich weeds as they are called, 

 give place sooner or later to those of the barrens. Nor is this the only evidence of a natural rotation. 

 After a season when the roots of grasses have produced a mat of vegetable fibres, is it not well known 

 that the meadow becomes infested with wild onions, buttercups, (Rantmcuhis,) thistles, and other 

 weeds, which, if not exterminated, soon overwhelm the grasses ? Hence the prudent husbandman 

 adds ashes or lime, and scarifies his meadows ; for by these means the roots are rapidly decomposed, 

 and the soil brought back to a state of composition favorable to the development of grasses : or if he 

 be conducting a rotation, he ploughs the meadow, and thus acquires by art a natural coat of manure, 

 of great service to such cultivated crops as, like the Chenopodiaceae, require a soil rich in organic 

 matters. 



II. Explanation of the foregoing natio-al rotation. 



The difficulty of making certain plants grow after each other in the same soil, was said to arise 

 from the mutual repulsion of plants, and explained by Von Humbolt, Plenk and De Candolle, 

 by reference to the experiments of Brugman and Macaire. These naturalists discovered that the 

 root of a plant growing in Avater, throws out a dark mucilaginous fluid which they called its excre- 

 ment. Thus the excrementitious deposit of any plant is supposed to be inimical to the growth of its 

 species, and also to some others ; but may on the other hand be of service to an entirely different 

 family. De Candolle saw in these reputed facts the explanation of rotations, which he therefore 

 resolved into die art of discovering such a succession of crops, that each might flourish on the organic 

 remains of its predecessors. Clean fallows were also commended as a means of hastening the decom- 

 position of excrementitious matters. 



But it is neither satisfactorily shown that excrementitious matters accumulate in the soil nor 

 that they are inimical to the growth of the species. Macaire, Braconnot and others have failed 

 to obtain positive evidence of such dejections, when a soil was employed instead of water, and 

 Alfred Gyde states that plants are benefitted by watering with a solution of their excrementitious 



