358 PLANT-LIFE 



tatively active by means of their foliage ; in the autumn 

 the leaves fall and vegetative activity is arrested; in 

 winter the trees are destitute of foliage and assume a 

 pronounced xerophytic form, in which vegetative activity 

 is hardly existent. Herbaceous perennials lose all their 

 aerial vegetative parts in winter; they die down to the 

 ground, and endure the cold season by means of their 

 underground organs, which are xerophytic for the time 

 being. There are xerophilous tropophytes which, during 

 their summer activity, are adapted to life in such physic- 

 ally dry places as sand-dunes, or such physiologically 

 dry situations as salt-marshes. Most of our annuals and 

 biennials, as well as our deciduous trees and herbaceous 

 perennials, are mesophilous tropophytes, being in sum- 

 mer adapted to existence in intermediate conditions as 

 to water-environment. The annual plants, which pass 

 through all stages of existence in a single season may 

 be said to endure winter in the form of the seeds they 

 produce. Seeds are most pronouncedly xerophytic. 

 Marsh and bog plants are hygrophilous tropophytes, 

 being in their summer condition in a constant moist 

 environment to which they are well adapted. 



In order that plant-communities may be aptly classi- 

 fied and described in an ecological survey, certain terms 

 are made use of, and in the study of plant-ecology it is 

 essential that the student should appreciate the signifi- 

 cance of these terms. Ecology, be it noted, is in its 

 infancy; it is by no means stereotyped, and any con- 

 clusions as yet reached must be regarded as more or less 

 tentative. There are differences of opinion among 

 investigators in various countries as to the best terms to 

 employ in relation to plant-communities. The terms 



