486 ECOLOGY 



and, in the case of physiographic ecology, of taxonomy, physiography, 

 geology, and meteorology as well. Partly because of its complexity 

 and partly because of its imperfect organization, it is impossible to 

 present all its materials, even in elementary fashion, within the com- 

 pass of such a book as this. Consequently, it has been thought wise to 

 center attention upon morphological and physiological ecology, since 

 this aspect is more in harmony with the other parts of the book and is 

 better suited for elementary presentation to students of botany. 



Terms. While most of the terms employed are defined where first 

 introduced or used extensively, a few are of such general employ- 

 ment as to justify consideration here. Since water generally is regarded 

 as the most important factor, the commonest ecological classification 

 of plants is into hydrophytes, mesophytes, and xerophytes. Hydro- 

 phytes are plants of water or of wet soil; xerophytes are plants of dry 

 areas, such as deserts, dry rocks, and dry sand; and mesophytes are 

 plants of soils intermediate as to moisture. The term xerophyte has 

 caused much confusion, because many plants growing in soils of inter- 

 mediate or abundant moisture have structures resembling those found 

 in plants of dry soils and climates. The most conspicuous instances of 

 such plants are found in alpine and arctic regions, in salt marshes, and 

 in peat bogs, and such terms as bog xerophytes and salt marsh xerophytes 

 are in common usage. The current theory is that deserts, rocks, and 

 sandy areas are physically dry, that is, lacking in water : whereas salt 

 marshes, peat bogs, and many alpine and arctic habitats are physiologi- 

 cally dry, that is, the water even if abundant is unavailable. In arctic 

 and alpine regions this is because of low temperatures, and in salt 

 marshes it is because of the high osmotic pressure of the medium, 

 while in the case of bogs various theories have been suggested. 



The term variation is employed in its broader sense of difference or 

 diversity, rather than in its narrower evolutionary sense of " deviation 

 from type." Ecological variation, or diversity produced through the in- 

 fluence of external factors, frequently is contrasted with taxonomic varia- 

 tion, where there is had in mind diversity between allied forms rather 

 than actual modification; that is, the former is necessarily dynamic, 

 whereas the latter may be relatively static. Frequent use is made of the 

 term external factors, as opposed to internal or inherent factors. By 

 the former are understood such factors as are outside the plant, and by 

 the latter, such factors within the plant as are hereditary. There is 

 another class of factors, known as correlative factors or correlations, 



