FACTORS 19 



Classification of Factors 



29. The nature of factors. The factors of a habitat are arranged in two 

 groups according to llieir nature: (i) physical, (2) biotic. In the strict 

 sense, the physical factors constitute the habitat proper, and are the real 

 causative forces. No habitat escapes the influence of biotic factors, how- 

 ever, as the formation always reacts upon it, and the influence of animals 

 is usually felt in some measure. Physical factors are further grouped into 

 (i) climatic and (2) edaphic, with respect to source, or, better, the medium 

 in which they are found. Climatic, or atmospheric factors are humidity, 

 light, temperature, wind, pressure, and precipitation. Axiomatically, the 

 stimuli which they produce are especially related to the leaf. Edaphic or 

 soil factors are confined to the -soil, as the term denotes, and are im- 

 mediately concerned with the functions of the root. Water-content is by 

 far the most important of these; the others are soil composition (nutrient- 

 content), soil temperature, altitude, slope, exposure, and surface. The last 

 four are of a more general character than the others, and are usually re- 

 ferred to as physiographic factors. Cover, when dead, might well be placed 

 among these also, but as it is little different from the living cover in effect, it 

 seems most logical to refer it to biotic factors. 



30. The influence of factors. While the above classification is both ob- 

 vious and convenient, a more logical and intimate grouping may be made 

 upon the influence which the factor exerts. On this basis, factors are 

 divided into (i) direct, (2) indirect, and (3) remote. Direct factors are 

 those which act directly upon an important function of the plant and produce 

 a formative effect: for example, an increase in humidity produces an im- 

 mediate decrease in transpiration. They are water-content, humidity, and 

 light. Other factors have a direct action: thus temperature has an im- 

 mediate influence upon respiration and probably assimilation also, but it is 

 not structurally formative. Wind has a direct mechanical effect upon woody 

 plants, but it does not fall within our definition. Indirect factors are those 

 that affect a formative function of the plant through another factor; thus 

 a change in temperature causes a change in humidity and this in turn calls 

 forth a change in transpiration; or, a change in soil texture increases the 

 water-content, and this affects the imbibition of the root-hairs. Indirect 

 factors, then, are temperature, wind, pressure, precipitation, and soil compo^ 

 sition. Remote factors are, for the most part, physiographic and biotic: 

 they require at least two other factors to act as middlemen. Altitude affects 

 plants through pressure, which modifies humidity, and hence transpiration. 

 Slope determines in large degree the run-off during a rain-storm, thus 



