61 



Weather conditions which favor the growth of large green plants 

 also favor the multiplication and work of the microbes. When man 

 cultivates soil he admits oxygen which stimulates microbial activity, 

 like opening the draft 011 a furnace. At the same time, the changes 

 in soil conditions resulting from cultivation destroy some of the 

 microbe species, and a new association is established. By over-culti- 

 vation, fertility often is made available from the humus faster than 

 it can be used by crops ; so that the excess may be lost by leaching 

 or, in the case of nitrates, may escape into the air. Thus even these 

 microscopic life forms become an object of management by man. 

 If we insist on having a social order such as we have developed, 

 minute and painstaking attention to every detail is imperative if we 

 are to maintain that culture permanently. The only other road leads 

 back to a primitive environment. As a general rule in nature, until 

 an undisturbed maximum is reached, life improves its home. Man, as 

 a part of nature, should logically follow this cue. 



Life Changes Forms. As developed briefly in the preceding chap- 

 ter, a changing environment made possible the survival of many varia- 

 tions and mutations, and their establishment as new species. These 

 new species occupied the various environments as they became avail- 

 able. Usually, when a new species appears in a plant community it 

 affects the vegetation already present. It may be more efficient in ex- 

 tracting water or minerals from the earth and thus starve out nearby 

 species. It may grow tall and provide enough shade to kill shorter 

 plants or prevent their reproduction. 



Plants (and animals) do alter the environment, often preparing it 

 for more highly organized life forms. The latter may then make it 

 impossible for their beneficent predecessors to live there. 



Life is Interrelated : The various plants and animals are not only 

 adjusted to the conditions of the mineral and climatic environment, 

 they are equally dependent on a friendly community of other species 

 of plants and animals. 



Life is marked by both variety and organization. The individual 

 is organized, no matter how simple or complex its structure. Animal 

 species are organized, as to range, feeding areas and habits, reproduc- 

 tive mechanics, etc., some remarkably so, as the bees and the ants. 



Equally remarkable relationships exist among plants. Communi- 

 ties of various species, like people, may be compatible or may not. If 

 not, one or more has to go. 



The plants in an association have many needs in common, which 

 enable all to live in the same general environment. Yet, many species 

 of the group vary in their needs, and these differentiated needs may 

 be met by other species in the association. Oak and hickory trees, in 

 one association, provide shade for those of the ferns, mosses, and other 

 plants which are intolerant of light. The forest floor holds the 

 abundant moisture needed by many low growing plants, and provides 

 the organic matter required by the numerous fungi. The young trees, 

 growing slowly in the subdued light of the understory, offer no 



