12 Richard H. Boerker 
the soil. He further subdivides climatic factors into humidity, 
light, temperature, wind, pressure and precipitation. The edaphic 
factors are subdivided in a similar way into water content, soil 
composition, soil temperature, altitude, slope, exposure, and 
surface. 
In glancing over this classification it becomes at once obvious 
that all of these factors cannot affect the plant directly. Many 
of those enumerated are in themselves very complex in their 
nature. For example, slope, aspect, altitude, and surface could 
each be subdivided into component factors, but if this is done it 
will be seen that they resolve themselves into those factors men- 
tioned above which are not divisible. In other words there are 
about three master factors which are able to affect plant life 
directly, and all others are combinations of these. There is no 
better way to bring out this idea than to give Clements’ (1) 
classification based upon the influence which each of these factors 
may exert on plant life. He classifies factors into those that have 
a direct bearing upon plant life, those that have an indirect bear- 
ing, and those that have a remote bearing. Direct factors are 
only those which produce qualitative structural changes in the 
plant itself. Furthermore, the classification of habitat forms and 
plant formations is based upon them, which fact merely em- 
phasizes that they are fundamental. Indirect factors are those 
that affect a formative function of the plant through another 
factor ; and remote factors are those which are physiographic or 
biotic in nature and must operate through at least two other 
factors in order to produce a structural change in the plant. This 
classification is as follows: 
Direct Factors Indirect Factors Remote Factors 
Water content Temperature Altitude 
Humidity Wind Slope 
Light Pressure Exposure 
Precipitation Surface 
Soil composition 
Soil temperature 
