. FACTORS 19 
CLASSIFICATION OF FACTORS 
29. The nature of factors. The factors of a habitat are arranged in two 
groups according to their nature: (1) 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 
(1) climatic and (2) edaphic, with respect to source, or, better, the medium 
in which they are found. Climatic, or atmospheric facters 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 (1) 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 doés 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 
