ing been reduced to about 16 pounds on the unmanured plot, and less than 27 pounds on the plot with mineral manure but without nitrogen, very large crops of red clover were grown containing about 300 pounds of nitrogen per acre.
If attention is directed exclusively to the root-tubercles of plants and the roots to which they are attached, it is difficult to understand the manner in which the free nitrogen of the air permeating the soil is made available by the microbes for the nutrition of the more highly organized hosts with which they are associated; but the problem is simplified when we take into consideration the interdependent relations of living organisms arising from their habits, and different requirements in their processes of nutrition.
The influence of cats on the growing of clover seed, as pointed out by Darwin, furnishes a good illustration of dependent relations in the struggle for existence. Cats prey on field-mice that destroy the nests of humble-bees, and the bees are known to be important factors in the fertilization of the clover plant. Quite as marked relations of dependence have been observed among microbes, but the sequence of organisms may be brought about by a different process.
In the ordinary processes of putrefaction we find an orderly succession of living organisms engaged in the work of disintegration in which relations of dependence are clearly manifest. The microbes that initiate the putrefactive process appropriate the materials required for their own growth and multiplication, and the residual mass soon becomes better fitted for the nutrition of other species which succeed them. These are, for similar reasons, succeeded by other forms that are better adapted to the changed conditions, and a series of organisms, of diverse habits, is required to reduce the organic compounds to their elements. Each species performs a specific rôle, "the earlier ones preparing the pabulum, or altering the surrounding medium, so as to render it highly favorable to a succeeding form" while their own activities are checked by the changed conditions.
The term symbiosis, as now used, is limited to the immediate and direct relations of certain species that are mutually beneficial in their processes of nutrition and growth; but this interdependence of vital activities and interests, in many cases at least, seems to extend to more remote relations through a series of organisms, each of which may have an influence on the well-being of the others. An increased growth of clover in a nitrogen-free soil has been obtained by seeding it with an extract from a root-crop soil; and this, in connection with the facts already presented, is certainly suggestive in explaining the advantages arising from crop rotations.
