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' (i) 

 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 



