CHAPTEE XXXVI 

 ECOLOGICAL GROUPS AND THEIR CHARACTERISTICS * 



435. Ecological grouping of plants. The ordinary classifi- 

 cation of plants, as set forth in Part II, is based, as far as 

 possible, on their actual relationships to each other. But when 

 plants are considered ecologically they are grouped according to 

 their relations to the world about them. They may, therefore, 

 be gathered into as many (or more than as many) different 

 groups as there are important factors influencing their modes of 

 life. We may, for instance, classify plants as light-loving and 

 shade-loving, and so on. 



The most important consideration in classifying seed plants 

 on ecological grounds is based on their requirements in regard 

 to water. Grouped with reference to this factor in their life all 

 plants may be designated as : 



1. Hydrophytes, or water-inhabiting or water -tolerating plants. 



2. Xerophytes, or drought-tolerating plants. 



3. Mesophytes, or plants which thrive best with a moderate supply 



of water. 



These three groups do not fully express all the relations of plants 

 to the water supply, so two others are found convenient : 



4. Tropophytes, or seasonal plants which are hydrophytes during part 



of the year and xerophytes during another part. 1 



5. Halophytes, or salt-marsh plants and " alkali " plants, species 



which can flourish in a very saline soil. 



* To THE INSTRUCTOR : If it is necessary to shorten the treatment of this 

 subject, the latter part of the chapter, beginning with Sec. 442, may be 

 omitted, 



1 The plants which E. Warming, one of the foremost authorities, classes 

 as mesophytes are many of them grouped by another great authority, 

 A. F. W. Schirnper, as tropophytes. 



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