THE PHYSICAL BASIS 37 



of them often show such a relation. Thus an abundance of water means a 

 lack of oxygen, and a deficit of water a strong soil solution. Habitats deficient 

 in light rarely show a lack of water or nutrients, though the oxygen-content of 

 the soil may be low also. In practically all herbaceous communities, light 

 is usually at the maximum, and the limiting factor must be sought in the soil. 

 Hence, a careful scrutiny of many habitats narrows the search for limiting 

 factors to a single one, and it is then possible to proceed at once with the 

 quantitative correlation of factor and indicator. 



It must also be recognized that some factors limit plant response in conse- 

 quence of an excess. This is true to some extent of solutes and water, but 

 not of light or oxygen in nature. Even with the former, while the excess 

 definitely limits or at least characterizes the plant's activity, the corresponding 

 deficit of water in saline soils and of oxygen in wet ones or in ponds also plays 

 a significant r61e. For water and solutes, it is probably more accurate to say 

 that the extremes, either excess or deficiency, act as limits. While there are 

 statements to the effect that full sunlight is directly injurious to many species, 

 there is little or no conclusive evidence. This feeling has been based largely 

 upon Bonnier's work with alpine dwarfing, which has not been confirmed by 

 similar studies in the Rocky Mountains. 



After eliminating the large groups of species which owe their indicator 

 character to the limiting action of water, solutes, oxygen, or shade, there 

 remains a much larger group of sun mesophytes which bear no such distinctive 

 impress. In a mesophytic habitat the four factors are present in a more or 

 less balanced optimum. No one exists in marked deficiency or excess. Yet 

 it has been demonstrated experimentally that a moderate increase in any 

 one of the factors will be reflected in an increase of growth. Each factor in 

 reality exerts a circumscribed limiting action as an outcome of competition 

 between the plants. The various effects, however, are so moderate and so well- 

 balanced that it is practically impossible to separate them. While water is 

 usually paramount and light often the least important factor in the competi- 

 tion between sun mesophytes, all four factors show a limiting action in at least 

 a small degree. In spite of its apparent lack of a distinctive impress, a meso- 

 phyte is as much the product of its habitat as the well-marked hydrophyte or 

 halophyte, and serves equally well as an indicator. 



Climatic and edaphic factors. — The factors of climate and soil are so intri- 

 cately interwoven in the habitat as to discourage analysis. For many reasons 

 it is better to ignore such a distinction as of little or no significance to the 

 plant and to fix the attention upon the cause-and-effect relation of one factor 

 to another, quite independently of its location. This will reveal clearly two 

 basic facts, namely, that the habitat is a unit and that the action of this unit 

 is focussed upon plant and community by one or two limiting factors. The 

 relation of the plant to water makes it evident that the distinction is merely 

 one of classification which has no real significance to the plant. Water- 

 content as a direct factor resident in the soil is directly or indirectly the result 

 of precipitation, a climatic factor, and is profoundly affected by humidity, a 

 climatic factor which it also influences. Its availability is determined by 

 soil-texture, solutes, and oxygen, all soil factors, and by temperature, which 

 belongs to both soil and air, though in origin it is climatic. The baffling nature 

 of the distinction has been well shown by Raunkiaer (Plant Succession, 1916:6). 



