RESPONSE OF THE PLANT TO ITS SURROUNDINGS 277 



3. Which of the five ecological factors mentioned in paragraphs 311- 

 315 has probably most largely influenced their distribution? 



4. What is the prevailing character of the soil in your neighborhood ? 



5. Is your climate moist or dry ? Warm or cold ? 



6. Can you trace any connection between these factors and the pre- 

 vailing types of vegetation? 



II. PLANT ASSOCIATIONS 



MATERIAL. The subject is not well suited to laboratory work, though, 

 if time permits, it is recommended that a detailed study be made of at 

 least one typical hydrophyte, halophyte, and xerophyte plant. Some 

 good examples are : (1) Hydrophyte : pond weed, waterlily, pipe wort (Erio- 

 caulon), bladderwort, arrowhead (Sagittarid) ; (2) Halophyte : sea la vender, 

 sea rocket, sea lettuce, water hyacinth ; (3) Xerophyte : cactus, century 

 plant, pineapple, stonecrop, purslane, lichen. 



316. Modes of grouping. Plants group themselves in 

 their favorite habitats, 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 possession 

 of large areas, like the coarse grasses that spread over certain 

 salt marshes, or the pines that formerly constituted the sole 

 forest growth over extensive regions in North Carolina and 

 Maine. Exclusive growths of this kind over limited areas 

 are sometimes called plant colonies, and the individuals com- 

 posing them belong, as a general thing, to the hardy, pushing 

 sort known as " pioneers," which are among the first to take 

 possession of new soil and force their way into unoccupied 

 territory. But more usually we find a great diversity of 

 forms brought together by their common requirements as 

 to shade, soil, moisture, and other external conditions. 



Any well-defined assemblage of plants, whether of one kind 

 or many, originating in such a common response to the same 

 influences, is called a formation. These associations are va- 

 riously classed, according to the nature of their habitat, 

 as salt water, fresh water, sand hill, swamp, bog, river bot- 

 tom, or such other kinds as their ecological character may 



