MESOPHYTE ASSOCIATIONS. 235 



These new associations have been formed by the introduc- 

 tion of weeds and culture plants. 



167. The two groups of associations. — Two very promi- 

 nent types of associations are included here under the meso- 

 phytes, although they are probably as distinct from one 

 another as are the mesophyte and xerophyte associations. 

 One group is composed of low vegetation, notably the com- 

 mon grasses and herbs ; the other is a higher woody vegeta- 

 tion, composed of shrubs and trees. The most character- 

 istic types under each one of these divisions are noted as 

 follows : 



A. Grass and herb associations. 



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. 



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

 low vegetation occurring beyond forest growth in arctic 

 regions, and above 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 prevents the 

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

 season is too short for them to reach a protected condition, 

 and their more exposed young structures are not in a posi- 

 tion to withstand the daily fall of temperature. 



These carpets of vegetation are notably fresh-looking, 

 indicating rapid growth ; green, indicating an abundance 

 of chlorophyll and great activity ; thick, as they are 

 mostly perennials, developed from abundant underground 

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



