26 G. D. FULLER AND A. L. BAKKE 



conditions and while I would not say that we must always 

 express our results numerically it is evident that where scientific 

 comparisons are undertaken we should be able to present phe- 

 nomena mathematically or in terms that are as definite and 

 unmistakable as numbers. In the field of biology, where there 

 are so many complexities of form and character, it is often a 

 difficult matter to measure, or estimate with exactness the 

 value of a single character and thus determine its significance. 



Ecology is one of the most difficult branches of botany in this 

 particular, not from any specific peculiarity, but because it em- 

 braces all the relations and all the difficulties of botany taken as 

 a whole. Thus every student of botany is concerned with 

 ecology because it includes plant behavior under all existing 

 conditions. 



The factor most frequently emphasized in ecology is that of 

 water. During the last forty years, especially during the last 

 quarter of the nineteenth century, plants have been classified 

 according to their water relations. They exhibit variations of 

 structure which serve to protect them against excessive trans- 

 piration and hence they exist in habitats where excessive evapo- 

 ration occurs, or where absorption of water is difficult, either 

 because of physical or physiological factors. 



These variations of structure are such things as (1) protective 

 coverings, (2) thick epidermis, (3) hypodermal tissue, (4) water 

 storage tissue, (5) coverings of hairs, (6) protective devices 

 for stomatal openings, (7) depression of stomata, (8) pits en- 

 closing the stomata, and (9) *a decrease in the area of leaf sur- 

 face exposed. 



Again it is apparent from the nature of the case that it is a 

 difficult matter to reach an exact conclusion based upon the 

 biological (ecological) characters of a single plant community. 

 The members of such a community existing under arid condi- 

 tions are some, of them present on account of acquired charac- 

 ters of one sort while others exist because of quite different 

 adaptations and we cannot obtain the quantitative value of a 

 single character or of a single group of characters. 



