372 GEXERAL BIOLOGY 



the distribution of water has largely determined the 

 grouping of terrestrial plants into natural societies: 



Hydrophytes — Plants accustomed to abundant avail- 

 able moisture. 



Mesophytes — Plants that live under intermediate condi- 

 tions. 



Xerophytes — Plants that live where the water supply- 

 is scanty, and that have deep roots, and many adaptations 

 for conserving the water supply. 



AVithin each of these groups the distribution of the mem- 

 bers in relation to each other — their mutual adjustment in 

 place — is determined more largely by exposure to light than 

 by any other single factor. Besides food, green plants must 

 have light, to supply the energy for growth that their 

 simple foods lack. This is especially true of the mesophyte 

 society, with its extraordinary diversity of size and form and 

 habitat. Be it forest, heath, or meadow, we always find it 

 dominated by a few relatively large species of great vegeta- 

 tive vigor. Around and between these, occupying the 

 interstices, and holding what soil and sunshine they can get, 

 are a host of lesser species, scattered, diversified and often 

 highly specialized as to their mode of performing particular 

 functions. It is among these that we find the most special 

 forms of plant-body and the most special devices for secur- 

 ing cross-pollination and seed distribution, etc. A few of 

 these plants of the undergrowth sometimes show a sort of 

 secondary dominance, their crowns forming imperfect foliage 

 strata at successively lower levels. Thus in the hard-wood 

 forests of our northern mountains there is often a top 

 stratum of crowns of maple, beech and birch at high altitude ; 

 a secondary stratum of the spreading tops of the hobble- 

 bush, a few feet above the ground, and a third stratum 

 of moss, carpeting the floor of the forest. Often in oak 

 woods farther south, there are successively lower strata of 



