318 ECOLOGY 



influence of a series of factors, topography, soil, and climate, 

 which will permit or favour the growth of certain species 

 to the exclusion of others, and such vegetation will have 

 a definite character. In other words, the nature of the 

 habitat must determine largely, not only the form, but the 

 kind of plant growing in it. The study of plants in relation 

 to their habitats is called ecology (from Gr. oikos= house 

 or habitat). 



Influence of water-supply on plant form. So important 

 is the water-supply to plants that, in proportion as the 

 amount available is large or small, they often develop 

 forms and structures suited to the conditions of such 

 habitats. In extreme cases this is so marked that special 

 names have been used to designate them. For example, 

 plants whose structural peculiarities enable them to grow 

 in water are known as aquatic plants or hydrophytes 

 (Gr. hydor = water, phyte = plant). Plants growing in 

 marshy ground, as on the sides of ponds, ditches, and 

 rivers, or in wet hollows in woods, are called hygrophytes 

 (Gr. hygros = moist). At the other extreme are plants 

 adapted to life in habitats with an uncertain water-supply 

 and under conditions favouring strong transpiration, e. g. 

 sand-dunes (Fig. 210), moors, and deserts. Such plants are 

 called xerophytes (Gr. xeros = dry), while plants growing 

 in a salt-marsh are known as halophytes (Gr. hals = salt). 

 Every gradation, however, is found between hydrophytes 

 and xerophytes, and it is impossible to draw a sharp line 

 between them, but it has been found convenient to speak 

 of plants adapted to habitats intermediate between the 

 two extremes as mesophytes (Gr. mesos = intermediate). 

 Included under the name mesophytes are plants very 

 varied in habit, form, and structure. Those which show 

 marked seasonal differences, e. g. plants with deciduous 

 leaves, which are mesophytes in summer and xerophytes 

 in winter, are called tropophytes (Gr. tropos = change). 



