HABITAT-FORMS. 65 



and combined halophytes with xerophytes, thus establishing the following 

 six groups: hydrophytes, mesophytes, hylophytes, poophytes, aletophytes, 

 and xerophytes. This division of mesophytes retained some idea of life-forms, 

 and it was later dropped (1902 : 166; 1907 : 183) for the consistent light 

 grouping of mesophytes into heliophyta, sciophyta, and scotophyta, correspond- 

 ing essentially to Schouw's classification into sun, shade, and darkness plants 

 (1823 : 166). A detailed classification of habitat-forms was made by Clements 

 (1902 : 5-14), in which light, solutes, aeration, and other factors were taken 

 into account, but with water-content as the primary basis. The 64 sub- 

 divisions were largely successional and physiographic; and this number can 

 be greatly reduced if factors alone are considered. This is essentially what 

 Warming has done in his most recent grouping of formations (1909 : 136), 

 which also represents much the best classification of habitat-forms up to the 

 present. His system is as follows: 



A. The soil (in the widest sense) is very wet, and the abundant water is available to the 



plant; the formations are therefore more or less hydrophilous: 

 Class 1. Hydrophytes (of formations in water). 

 Class 2. Helophytes (of formations in marsh). 



B. The soil is physiologically dry, i.e., contains water which is available to the plant only 



to a slight extent; the formations are therefore composed essentially of xerophi- 



lous species: 

 Class 3. Oxylophytes (of formations on sour (acid) soil). 

 Class 4. Psychrophytes (of formations on cold soil). 

 Class 5. Halophytes (of formations on saline soil). 



C. The soil is physically dry, and its slight power of retaining water determines the vege- 



tation, the climate being of secondary import; the formations are therefore likewise 



xerophilous : 

 Class 6. Lithophytes (of formations on rocks). 

 Class 7. Psammophytes (of formations on sand and gravel). 

 Class 8. Chersophytes (of formations on waste land). 



D. The climate is very dry and decides the character of the vegetation; the properties of 



the soil are dominated by cbmate; the formations are also xerophilous: 

 Class 9. Eremophytes (of formations on desert and steppe). 

 Class 10. Psilophytes (of formations on savannah). 

 Class 11. Sclerophyllous formations (bush and forest). 



E. The soil is physiologically or physically dry: 



Class 12. Coniferous formations (forest). 



F. Soil and climate favor the development of mesophilous formations: 



Class 13. Mesophytes. 



Modifications of Warming's system. — In making use of habitat-forms as 

 indicators in North American vegetation, a few modifications of the above 

 groups are desirable. These are perhaps further warranted by some advance 

 in ecological knowledge in the ten years since Warming made the following 

 statement concerning habitat-forms (1909 : 133) : 



"When endeavoring to arrange all land-plants, omitting marsh-plants, into 

 comprehensive groups, we meet with first some communities that are evidently 

 influenced in the main by the physical and chemical charaoters of the soil 

 which determine the amount of water therein; secondly, other communities 

 in which extreme climatic conditions and fluctuations, seasonal distribution of 

 rain and the like, decide the amount of water in soil and character of vegeta- 

 tion. In accordance with these facts, land-plants may be ranged into groups, 

 though in a very uncertain manner. The prevailing vagueness in Ulis group 



