MESOPHYTE ASSOCIATIONS 215 



These new associations have been formed by the introduc- 

 tion of weeds and culture plants. 



14G. The two groups of associations. — Two very prom- 

 inent types of associations are included here under the 

 mesophytes, 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 characteristic 

 types under each one of these divisions are noted as follows. 



Among the mesophyte grass and herb associations are 

 the "arctic and alpine carpets," so characteristic of high 

 latitudes and altitudes where the conditions forbid trees, 

 shrubs, or even tall herbs ; " meadows," areas dominated by 

 grasses (Fig. 197), the prairies being the greatest meadows, 

 where grasses and flowering herbs are richly displayed (Fig. 

 198) ; " pastures," drier and more open than meadows. 



Among the woody mesophyte associations are the " thick- 

 ets," composed of willow, alder, birch, hazel, etc., either 

 pure or forming a jungle of mixed shrubs, brambles, and 

 tall herbs ; " deciduous forests," the glory of the temperate 

 regions, rich in forms and foliage display, with annual fall 

 of leaves, and exhibiting the remarkable phenomenon of 

 autumnal coloration (Figs. 194-196) ; " rainy tropical for- 

 ests," in the region of trade winds, heavy rainfalls, and 

 great heat, where the world's vegetation reaches its climax, 

 and where in a saturated atmosphere gigantic jungles are 

 developed, composed of trees of various heights, shrubs of 

 all sizes, tall and low herbs, all bound together in an inex- 

 tricable tangle by great vines or lianas, and covered by a 

 luxuriant growth of numerous epiphytes (Fig. 199). 



15 



