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 
Mareriau. — 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: pondweed, waterlily, pipewort (Erio- 
caulon), bladderwort, arrowhead (Sagittaria) ; (2) Halophyte : sea lavender, 
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 
