PLANT SOCIETIES 241 



PLANT SOCIETIES 



MATERIAL. A specimen of pipewort {Eriocaulori), Sagittaria, 

 pondweed, or other succulent water plant, and a cactus of some kind. 

 The common prickly pear (Opuntia) is the one used in the text. 

 City schools should have a small aquarium ; a few water plants can be 

 kept in jars. 



348. Principles of Subdivision. Plants group them- 

 selves into societies not according to their botanical rela- 

 tionships, but with regard to the predominance of one or 

 more of the ecological factors that influence their growth. 

 Sometimes one or two species will take practical posses- 

 sion of large areas, like the coarse grasses that spread 

 over certain salt marshes, or the pines that formerly con- 

 stituted the sole forest growth over extensive regions in 

 North Carolina and Maine. But more usually we shall 

 find a great diversity of forms brought together by their 

 common requirements as to shade, soil, moisture, etc. 

 These societies are, of course, purely artificial, and any 

 of the factors named in Sections 341-346, or others of a 

 different kind, may be made the basis of their classifica- 

 tion. They might be grouped, for instance, according to 

 the soil in which they grow, or according to origin, whether 

 cultivated, wild, native, introduced, etc., as best suited the 

 purpose of the classification in each case. The moisture 

 factor, however, has been generally agreed upon by bot- 

 anists as the one most convenient for ordinary purposes. 

 Upon this principle plants are divided into three great 

 groups : 



Hydrophytes, or water plants, those that require abun- 

 dant moisture. 



Xerophytes, or drought plants, those that have adapted 

 themselves to desert or arid conditions. 



Mesophytes, plants that live in conditions intermediate 

 between excessive drought and excessive moisture. To 

 this class belong most of our ordinary cultivated plants 

 and the greater part of the vegetation of the globe. 



ANDREWS' S EOT. 1 6 



