232 PLANT RELATIONS. 



These new societies have been formed by the introduction 

 of weeds and culture plants. 



102. The two groups of societies. — Two very prominent 

 types of societies are included here under the mesophytes, 

 although they are probably as distinct from one another as 

 are the mesophyte and xeroiDhyte societies. One group is 

 composed of low vegetation, notably the common grasses 

 and herbs ; the other is a higher woody vegetation, com- 

 posed of shrubs and trees. The most characteristic types 

 under each one of these divisions are noted as follows. 



A. Grass and herd societies. 



It should not be inferred from this title that most 

 grasses are not herbs, but it is convenient to consider 

 grasses and ordinary herb forms separately. 



103. Arctic and alpine carpets. — These are dense mats of 

 low vegetation occurring beyond forest growth in arctic 

 regions, and aliove the tree limit in high mountains. These 

 carpet-like growths are a notable feature of such regions. 

 In such positions the growing season is very short, and the 

 temperature is quite low at times, especially at night. It 

 is evident, therefore, that there must be provision for rapid 

 growth, and also for preventing dangerous radiation of 

 heat, which might chill the active plant below the point of 

 safety. It is further evident that the short season and the 

 low temperature form a combination which ])rcvents the 

 growth of trees or shrubs, or even tall herbs, because the 

 season is too short foi' them to reach a protected condition, 

 and their more exj^osed young structures are not in a posi- 

 tion to witlistaiid the daily fall of temperature. 



These ciu-pots of vegetation are notably fresh-looking, 

 indicating rapid growth ; green, indicating an abundance 

 of chlorophyll and great activity ; thick, as tliey ai'c 

 mostly perennials, developed from abundant underground 

 structures ; low, on account of the short season and low 



