126 PLANT SOCIOLOGY 



The first step toward an ecological understanding of water economy 

 leads to a grouping of plants according to water balance into the 

 following graded series: 



1. Hydatophytes or water plants. 



2. Hygrophytes, moisture-loving species with favorable water economy. 



3. Mesophytes, species with medium water relations. 



4. Xerophytes, drought plants, with small water requirement. 



Most of the representatives of a group with a definite water rela- 

 tion show many similarities in the form and structure of the organs of 

 transpiration, as well as in the finer structure of the internal tissues. 

 These structural arrangements seem to indicate the vital activity of 

 the species in adjusting itself to the environment. They are therefore 

 considered as adaptations to the conditions of the habitat. Hygro- 

 phytes have morphological devices that permit the free loss of water. 

 Xerophytes on the contrary have many structural characteristics which 

 serve to reduce transpiration and more particularly cuticular tran- 

 spiration, such as reduction of surface in relation to volume, reduction 

 of the intercellular spaces, thick outer epidermal walls with much 

 cutin and heavy cuticle, abundance of wax in the epidermis, and 

 coatings of wax, depression of stomata, covering of leaves with dead 

 hairs, frequently a water-storage system, and an extensive conductive 

 and absorptive system (Fitting, 1926, p. 18). 



This teleological exposition, although fundamentally right, has 

 received much criticism and correction from modern experimental 

 physiology. 



The prevalent misunderstanding has been undoubtedly intensified 

 by the indefinite and ambiguous definitions of the concepts hygrophyte 

 and xerophyte. Whereas some writers mean plants with hygro- 

 morphic or xeromorphic adaptations, others designate thereby species 

 which occupy wet or dry habitats. At present it is unanimously agreed 

 that every species must be examined by itself before judgment can be 

 passed upon its water relation. The visible morphological arrange- 

 ments for resistance to drought are far less important in their effects 

 than the purely physiological conditions. Consequently, plants of 

 pronounced xeromorphic structure, like the Ericaceae of our moors and 

 many Cyperaceae, grow persistently in moist or wet habitats. 



Ecologists of the Schimper school would explain all these phenom- 

 ena by the indefinite hypothesis of "physiological dryness." Mean- 

 while the careful investigations of Montfort (1918) and others have 

 shown that there is no such thing as physiological dryness, in the 

 Schimper sense, due to the difficulty of absorbing water from moors 



