=130. Plants.= As a dead cover, vegetation is a factor of the habitat proper, but it has relatively little importance, since it occurs regularly during the resting period. Its chief effects are in modifying soil temperature, and in holding snow and rain, and thereby increasing the water-content. By its gradual decay, moreover, it not only adds humus to the soil, but it thereby increases the water-retaining capacity of the latter also. The cover of living vegetation reacts upon the habitat in a much more vital fashion, exerting a powerful effect upon every physical factor of the habitat. The factors thus affected are distinctly measurable though it is often impossible to determine just how much of the factor is directly traceable to the vegetation. This is a simple problem in the case of most aerial factors, especially light, but it is extremely difficult for soil factors, such as water-content and soil texture. In the case of all habitats covered with formations, by far the great majority, it is impossible as well as unnecessary to separate the physical factors of the habitat proper from the reaction upon them which the plant covering exerts. Indeed, the great differentiation of habitats is largely due to the universal principle that in vegetation the effect or formation always reacts upon the cause or habitat in such a way as to modify it. As fundamental causes of succession, the discussion of the various reactions of vegetation is reserved for another place. METHODS OF HABITAT INVESTIGATION =131.= The use of the various instruments previously described depends largely upon the preponderance of simple instruments or recording ones. The former necessitate a number of well-trained assistants; the latter require only a part of the time of one investigator. For the most satisfactory results, however, an assistant is all but indispensable. Since simple instruments are most easily obtained because of their cheapness, and are especially adapted to purposes of instruction, the method of using them will be described first, and then that of ecograph batteries. THE METHOD OF SIMPLE INSTRUMENTS [Illustration: Fig. 29. Series of stations: I, at Minnehaha; II, at Lincoln in the prairie formation.]