THE PHYSIOLOGICAL BASIS. 45 



A somewhat similar case is afforded by the evergreen shrubs. In spite of the 

 work of Kihlmann (1890 : 88, 105), it has been generally assumed that the 

 evergreen shrubs of bogs, such as Chamaedaphne, Andromeda, Vaccinium, 

 Ledum, etc., were xerophytes essentially similar in water relations to evergreen 

 shrubs of arid climates. Recently the experiments of Gates (1914 : 445) have 

 confirmed the conclusions of Kihlmann that while they are xerophytic, it is in 

 response to physiological dryness in winter, and that they do not indicate 

 aridity in such habitats during the summer. In fact, the summer indications 

 are rather those of deficient aeration. 



When growth is considered, the response of the same species to different 

 extremes of one factor or another is often very similar. E. S. Clements 

 (1905 : 93) has found in control experiments with Chamaenerium, Aquilegia, 

 and Anemone that extremes of any factor which are not optimum for the species 

 tend to dwarf plants growing in them. The general principle has been formu- 

 lated as follows by Clements (1905 : 105) : 



"When a stimulus approaches either the maximum or minimum for the 

 species concerned response becomes abnormal. The resulting modifications 

 approach each other and in some respects at least become similar. Such 

 effects are found chiefly in growth, but they occur to some degree in structure 

 also. It is imperative that they be recognized in nature as well as in field and 

 control experiment, since they directly affect the ratio between response and 

 stimulus." 



This applies with especial force to the recognition of indicators, since their 

 value depends primarily upon the close correspondence between response and 

 the causative factor. 



Phy tometers. — The best indicator of the nature of a habitat and of its practical 

 utilization is the particular plant or community concerned. This is axiomatic, 

 but it needs emphasis in connection with the experimental study of indicators. 

 Such study may be made by means of physical instruments, standard plants, or 

 the plants to be grown as a natural or artificial crop. The former is the simplest 

 of the three, the latter the most effective. The use of standard plants com- 

 bines the advantages of both to a large degree, and seems destined to undergo 

 extensive development during the next few years. The refinement of method 

 will lead to an increasingly wider range of possible standard plants, until it 

 includes a large number of the species of greatest importance in agriculture, 

 forestry, and grazing. Out of these will emerge a few species of broad powers 

 of adjustment and adaptation which can be used as measures over great areas, 

 such as between the associations of a climax formation or even between climax 

 habitats themselves. A number of species of this sort are already clearly 

 pointed out by their vast ranges and their vigorous growth in different regions. 

 Of the grasses, Bouteloua gracilis, B. racemosa, Stipa comata, and Andropogon 

 scoparius are perhaps the most promising, and among shrubs Rhus trilobata, 

 Cercocarpus parvifolius, Ceanothus velutinus, and Rubus strigosus. Of the 

 trees, aspen is the best, with Pinus ponderosa and Pseudotsuga mucronata as the 

 best of the conifers for the western half of the continent. As general stand- 

 ards, such weedy herbs as Helianthus annuus, Melilotus alba, and Brassica 

 nigra are most useful. The most satisfactory cultivated plants are yet to be 

 determined, but wheat, corn, and beans have obvious advantages. 



