MESOI'lIVTE S()(.'1ETIES 215 



These new societies have beeii formed by tlie introduction 

 of weeds and culture plants. 



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

 types of societies are included here under the mesophytes, 

 altliough they are probably as distinct from one another as 

 are the mesophyte and xerophyte societies. One group is 

 composed of low vegetation, notably the common grasses 

 and herbs ; the other is a higher woody vegetation, comj^osed 

 «f shrubs and trees. The most characteristic types under 

 each one of these divisions are noted as follows. 



Among the mesophyte grass and herb societies are the 

 " arctic and alpine carpets," so characteristic of high lati- 

 tudes and altitudes where the conditions forbid trees, shrubs, 

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

 the prairies being the greatest meadows, where grasses and 

 flowering herbs are richly displayed ; " pastures," drier and 

 more open than meadows. 



Among the woody mesophyte societies are the " thick- 

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

 j)ure or forming a jungle of mixed shrubs, brambles, and 

 tall herl)s ; " deciduous forests," the glory of the temperate 

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

 of leaves, and exhibiting the remarkable and cons2)icuous 

 phenomenon of autnmnal coloration; "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. (See Figs. 195, 

 197, 198, 199.) 



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