PROGRESS OF AGRICULTURAL SCIENCE. 499 



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 per- 

 meating the soil is made available by the microbes for the nutri- 

 tion of the more highly organized hosts with which they are asso- 

 ciated ; but the problem is simplified when we take into considera- 

 tion the interdependent relations of living organisms arising from 

 their habits, and different requirements in their processes of nu- 

 trition. 



The influence of cats on the growing of clover seed, as pointed 

 out by Darwin, furnishes a good illustration of dependent rela- 

 tions 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 disinte- 

 gration 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 rea- 

 sons, 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 role, " 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 activi- 

 ties 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 interdepend- 

 ence 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. 



