36 BASES AND CRITERIA. 



THE PHYSICAL BASIS. 



Direct and indirect factors. An adequate understanding of the habitat as 

 the cause of plant responses which serve as indicators must rest upon two 

 facts. The first of these is that the habitat is a complex, in which each factor 

 acts upon other factors and is in turn acted upon by them. The second is 

 that some of these factors are direct causes of plant response, while others 

 can affect the plant only through them. Water, light, solutes, and soil-air 

 are direct factors of the first importance because of their variation from habitat 

 to habitat. Other direct factors, such as carbon dioxid, oxygen, and gravity, 

 are negligible because of their constancy. Temperature is both direct and 

 indirect, but its indirect action through the water relation is usually the most 

 tangible. Wind, pressure, slope, exposure, soil texture, etc., are all indirect, 

 acting for the most part through water-content or humidity, or through tem- 

 perature upon these. 



Too much importance can not be given this distinction between direct 

 and indirect factors. The indicator value of every plant depends upon it 

 absolutely. A plant can only indicate a direct factor. But by the correla- 

 tion of the latter with factors which are modifying it, the indicator response 

 of the plant may be related to these. Thus, dwarfed herbs usually indicate 

 a lack of water. In alpine regions this lack is largely caused by excessive 

 transpiration and evaporation due to low pressure. As a consequence, dwarfs 

 are typical indicators of high altitudes and hence of alpine climates. By other 

 correlations of direct factors with causative processes, such as disturbance, 

 erosion, cultivation, etc., plants come likewise to be used as process or prac- 

 tice indicators. The true basis of all plant indicators is to be found in the 

 responses made to direct factors, especially water, light, solutes, and soil- 

 air. These once established, it becomes a simple matter to connect indica- 

 tors with any correlated factor or process. 



Controlling and limiting factors. ^It is evident that the factor in immediate 

 control of the behavior of plant or community must be a direct one. But the 

 latter may be profoundly affected by another factor in which the actual con- 

 trol may be said to reside. For example, montane timber-lines are often 

 determined by water, but the availability of the water-content is decided 

 by frost and its sufficiency by the wind. As indicated above, the immediate 

 control and hence the immediate indication must be sought among the few 

 direct factors, while the final control and indication will be found among the 

 indirect factors which exert a critical effect. 



All the direct factors of the habitat play a part in the responses of the plant, 

 but only those which vary widely in quantity leave a distinct impress upon it. 

 This is necessarily true, since such constant factors as carbon dioxid, oxygen, 

 and gravity produce fairly uniform responses, and consequently do not differ- 

 entiate species or communities. In the case of each individual plant or species, 

 its distinctive features are due to one of the variable direct factors. In prac- 

 tically all cases at least one of these will be deficient, with the result that it 

 becomes the limiting factor in the plant's development. This term is used in 

 an ecological sense and not in the physiological one employed by Blackman 

 (1905) and others. As a result the search for indicator correlations among 

 the four direct factors narrows itself to the one or two which are deficient. 

 Some of these factors regularly bear an inverse relation to each other and all 



