2 PLANT HABITS AND HABITATS IN THE 



Owing, however, in part to the difficulty in evaluating the biological 

 value of the accessory factors, such as light intensity, relative humidity 

 of the air, rate of evaporation, and temperature of the air, and in part 

 to the fact that such secondary factors may change in relative force 

 with changes in the amount of the rainfall, it is difficult to express 

 adequately what constitutes a " desert," or even the degree of aridity. 

 Nevertheless it is important to have some method of comparison. 

 Thus, the extremes in amount of rainfall have been used (MacDougal, 

 1914: 175), and the amount of evaporation for any given year has been 

 compared with the precipitation for the same year, and, finally, com- 

 parisons have been instituted between the moisture content of the soil 

 and the rate of evaporation of the air (Shreve, 1915:92). 



The intensity of the aridity has also been expressed biologically in 

 terms of the relative number of annuals in a region (Paulsen, 1912: 159). 

 It was not convenient in the present instance to use these methods, but 

 it appeared necessary nevertheless (mainly for convenience in reference) 

 to have some ready means of comparing one region studied with 

 another, and in the end the device was resorted to of using the rainfall 

 only. An arbitrary classification of regions based on the amount of 

 rain was consequently adopted, which is as follows: A region having 5 

 inches, or less, of rain annually is a desert; one with a rainfall between 5 

 and 10 inches is arid; and a region in which the amount of rain is be- 

 tween 10 and 15 inches is semi-arid. In all cases, therefore, in which 

 reference is made in the text to regions so designated the appropriate 

 rainfall will at once be understood. 



The study of plants in the field may be said to proceed mainly along 

 three lines, which, although more or less intermingled, are fundamentally 

 quite different. Thus, the leading emphasis can be placed on the 

 plants as species, and their occurrence (local as well as general) can 

 merely be catalogued; this is plant geography in a narrow sense. Or, 

 the mutual relationships of plants can be investigated ; this is one formal 

 branch of plant ecology. Or, finally, the investigation can take into 

 consideration mainly the relations of plants to the physical environ- 

 ment in which they are placed; this third phase of the general subject 

 is intimately related to experimental researches along lines suggested by 

 field observations and is not to be dissociated from laboratory studies; 

 this can be referred to as physiological plant ecology. It is the last 

 type of ecological research which the writer has had especially in mind 

 when making field studies, and though it has not been practicable to 

 carry out direct experiments on subjects suggested by the observations, 

 it has been of interest and profit to interpret the observations so far as 

 possible in the light of experimental results already accomplished on 

 analogous lines and with analogous plants by various researchers. 



In addition to viewing the living plants from a physiological stand- 

 point, another point of view has been of use, the comparative. In all 

 instances the plants observed have been studied in the light of the 



