MESOPHYTE ASSOCIATIONS. 243 



171. Thickets. — The mesophyte thickets are not so 

 abundant or impenetrable as the xerophyte thickets. They 

 seem to be developed usually as forerunners of forest vege- 

 tation. An illustration of this fact may be obtained by 

 noting the succession of plants which appear on a cleared 

 area. After such an area has been cleared of its trees, by 

 cutting or by fire, it is overrun by herbs that develop 

 rapidly from the seed. Sometimes these herbs are tall 

 and with showy flowers, as the so-called fire-weed or great 

 willow herb. Following the herb associations there is a 

 gradual invasion of coarser herbs and shrubby plants, 

 forming thickets, and finally a forest growth may appear 

 again. 



In arctic and alpine mesophyte regions the willow is 

 the great thicket plant, often covering large areas, but in 

 temperate regions willow thickets are confined to stream 

 banks and boggy places, being the characteristic hydro- 

 phyte thicket form. 



The upland and flood-plain mesophyte thickets of tem- 

 perate regions are different in character. For example, 

 the upland thicket of the Xorthern States very commonly 

 contains hazel, birch, and aspen as dominant plants ; while 

 the flood-plain thicket is apt to contain, in addition to 

 these, prominent growths of haws and wild crab-apples. 

 In this same region pure thickets frequently occur — that 

 is, thickets in which a single form is the prevailing type, 

 as pure hazel thickets on uplands, or pure haw thickets on 

 flood-plains. 



In the Southern States the plants enumerated above 

 may not be the characteristic mesophyte thicket plants. 

 For example, in Kentucky and Tennessee the dominant 

 thicket plants are persimmon, locust, redbud, and 

 sassafras. 



178. Forests of temperate regions. — Deciduous forests 

 are especially characteristic of temperate regions. The 

 deciduous habit, that is, the .habit of shedding leaves at a 



