RELATIONS BETWEEN PLANTS AND ENVIRONMENT 69 



species with species. The destruction of weeds in crop fields by 

 means of cultivation, chemicals, or flame, is required to control such 

 competition. The forester deals with "weed trees," which use 

 nutrients and sunlight without providing man with timber of much 

 value. Forest management suggests their removal, if better trees 

 can use the environment. 



In such a community it will also be observed that some species 

 get along quite amiably with others, even to the point of inter- 

 dependence. This is due to a difference or partial difference in 

 requirements. The relation between legume plants and nitrogen- 

 fixing bacterial plants has been mentioned. The lichen represents a 

 close partnership between algae and fungi. Species may live in 

 intermixture because their roots or their crowns are on different 

 levels. Some species can live in the shade of taller plants, which 

 may live in the shade of still taller plants which may need full sun- 

 light. 



The term association means those species which, through successive 

 stages of environmental development ending in a relative state of 

 balance, have come to dominate the community, temporarily or perma- 

 nently. Thus the beech-maple forest actually consists of scores of 

 plant species all dominated by the beech-maple association. 



Ecological Factors. Frequent mention has been made of various 

 factors involved in the life processes of plants such as soil, water, 

 atmosphere, sunlight and temperature. It has been noted that these 

 are altered, so far as plants are concerned, as the environment de- 

 velops, that these alterations permit the invasion of the habitat by 

 other species, and often result in the disappearance of once established 

 kinds. Insofar as these factors affect the structure and composition of 

 communities they are ecological. 



The ecological factors named are all functions of climate, in the 

 final analysis. Temperature, sunlight, and precipitation need no ex- 

 planation. Atmosphere is put in motion by heat from the sun, and 

 thus distributes water, heat and cold. Unsaturated atmosphere picks 

 up moisture, thus influencing soil humidity. Soil itself is a product 

 of climate working directly on minerals or indirectly through the 

 biologic forces of life and death. The water holding capacity of soil 

 is thus a function of and product of climatic action. There is a lesser 

 dependence on the nature or composition of the parent rock in deter- 

 mining what grows in a specific area. "Vegetation . . . most quickly 

 and strikingly expresses the character of the climate." 2 



In regard to a given species of plant in a given habitat, any one 

 of these factors might be critical. The morning glory is killed early 

 in autumn by frost. Orange groves are often killed by one severe 

 freeze. Lack of water in general keeps forest out of the plains, and 

 Buffalo grass out of the desert. The need for much water keeps 



2 Sears, P. B., This Is Our World. University of Oklahoma Press, Norman, 

 Okla., 1937, p. 180. 



