82 KINDS OF INDICATORS. 



nishing adequate proof of the paramount control of temperature in so far as 

 plants are concerned at least. It possesses the disadvantages of every system 

 erected upon a single factor, and emphasizes the basic truth that studies of 

 causes must be grounded upon experiment, and not merely upon field observa- 

 tions and meteorologic data. 



While there can be little or no question that every species has a climatic 

 maximum and minimum of temperature, this is known experimentally for 

 none of them. What is ordinarily observed in nature is, broadly speaking, 

 an optimum to which the plant is more or less confined by the action of com- 

 petition, water, and other factors. Theories of temperature control have 

 generally failed to realize the unique importance of the period of germination 

 and seedling establishment in determining the range and dominance of a 

 particular species. There is sufficient experimental evidence in the case of a 

 few dominants to suggest that many if not all of them can be extended beyond 

 their present northern and southern, as well as their altitudinal limits, by the 

 proper control of local conditions during the period of early ecesis. Moreover, 

 when the part played by water in many of the effects supposed to be caused 

 by temperature is adequately understood, it will be recognized that many of 

 the so-called temperature responses must be ascribed to the combined action 

 of the two. 



In accordance with the rule, the impress of temperature should be most 

 pronounced in climates where it is most extreme. These are arctic and alpine 

 regions, and the tropics and subtropics. However, the influence of water is 

 also pronounced in the first two, and over much of the other two. The dwarf 

 shrubs and perennial herbs of alpine and arctic regions have long been regarded 

 as undoubted responses to short seasons and low temperatures. But in the 

 case of some alpine plants at least, it is certain that dwarfing is due as much 

 or more to water than to temperature (Clements, 1907). It appears highly 

 probable that this is true of the dwarfing of trees at timber-line also. In the 

 latter case, the non-availability of the water-content is caused by freezing, 

 and the dwarfing might well be regarded as due to both the direct and indirect 

 action of temperature. A similar relation exists in tropical and subtropical 

 deserts, where the actual impress is largely due to water. The latter is pro- 

 foundly influenced by temperature, which appears to be in control of dis- 

 tribution to considerable degree, especially in the case of succulents (Shreve, 

 1911, 1914). 



If some weight be assigned to the indirect action of temperature, a con- 

 siderable number of species may be regarded as temperature indicators. 

 These are primarily alpine and arctic plants, and the succulents of hot desert 

 regions. The trees and shrubs of the boreal tree limit and of timber-line on 

 mountains are similar indicators, and this is true to some degree of those trees 

 which become shrubs as they extend downward into the deserts of the South- 

 west. The absence of certain life-forms and species as a consequence of frost 

 also constitutes a temperature indication of great importance. As a conse- 

 quence of the gradual change of temperature with latitude and altitude, 

 climax communities serve as the best of temperature indicators. They com- 

 bine the responses of both life-form and species on such a large scale that there 

 can be little question of the paramount control of temperature where its 

 extremes are concerned. Between the latter, climax dominants and com- 



