Page:Popular Science Monthly Volume 22.djvu/97

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experiments to determine the differences in the amount and constituents assimilated by plants of different botanical families, under similar conditions, and of the same plant under varying conditions. From these investigations, so far as they have been published, it appears that the chemical relations of the plant and soil are, to a great extent, determined by botanical and physiological conditions.

In the experiments with the mixed herbage of "permanent meadow," for example, it was noticed, even in the first years of the experiments, that "those manures which are most effective with wheat, barley, or oats grown on arable land—that is, with the gramineous species grown separately—were also the most effective in bringing forward the grasses proper in the mixed herbage; and again, those manures which were the most beneficial to beans or clover, most developed the leguminous species of the mixed herbage, and vice versa."

In the produce grown continuously without manure the average number of species was forty-nine. Of these seventeen are grasses, four leguminous species, and twenty-three of other orders. By weight the grasses averaged sixty-eight per cent, leguminous species nine per cent, and species of other orders twenty-three per cent.

In the produce of the plot most heavily manured and yielding the heaviest crops, the average number of species was nineteen; of which twelve to thirteen were grasses, one only (or none) leguminous, and five or six only of other species. By weight the grasses averaged about ninety-five per cent, the leguminous species less than 0·01 per cent, and other orders less than five per cent.

On the plot receiving annually manures that are of little avail for gramineous crops grown separately in rotation, but which favor beans or clover so grown, the average number of species was forty-three, of which seventeen were grasses, four leguminous, and twenty-two belonging to other orders. But by weight the grasses averaged but from sixty-five to seventy per cent, the leguminous species nearly twenty per cent, and all other species less than fifteen per cent.

The "struggle for existence" and the "survival of the fittest," therefore, determine the character of the species contained in the produce under the conditions, and the chemical composition of the crop varies accordingly. With an increase of the leguminous produce the nitrogenous constituents are increased, and with a decrease in the leguminous produce the nitrogenous constituents are diminished.

Experiments with leguminous, gramineous, and other families of plants were made for several years in succession, at Rothamsted, to determine whether plants assimilate free or uncombined nitrogen.

The relations of nitrogen to the growing plant and to the soil and the sources of the nitrogen of vegetation have been prominent subjects of investigation in all the Rothamsted field-experiments.

It is not my purpose, in this connection, to discuss the various theories of vegetable growth, or to give an account of the many contro-