194 BOTANY 
Among the special adaptations of the family are 
the simple but effective device for cross-pollination, 
which will be fully deseribed in another place; the 
massing together of the flowers to form conspicuous 
heads; the protection of the inflorescence by means of 
an involuecre, and the development of the tuft or 
pappus of hairs, that in so many species serves to 
distribute the seeds. The rosette form, too, is common 
in this family and constitutes one of the strongest 
weapons of the daisy, dandelion, catsear and many 
other members of the order. 
Already we have seen how, owing to special 
adaptations of form, habit, and structure, certain 
plants are suited to a special environment. A number 
of factors determine the classes of plants that shall 
survive in any particular region, but by far the 
most important of these is the water factor. Exposure 
to wind, shade, temperature, the nature of the goil, 
the presence or absence of certain plant foods, are, 
as a rule, of minor importance. Since plants suited to 
a particular environment will oust all others that are 
not so well equipped as themselves to cope with the 
special conditions under which they are living, 
different plant societies have arisen, comprising certain 
typical groups, which in like situations show a remark- 
able similarity throughout the world. We have seen 
that scarcity of water is the chief factor that deter- 
mines what kinds of plants shall form any particular 
group. At the one extreme we have hydrophytes (Gk. 
hydor, water, and phyton, a plant), plants which, like 
the water lily, live in the water; and at the other 
xerophytes (Gk. zeros dry), plants which, like the 
bracken fern, are suited to dry conditions and 
surroundings. 
Mesophytes (Gk. mesos, middle) are plants adapted 
to an environment in which there is a good but not 
excessive supply of water. 
