MESOPHYTE SOCIETIES. 239 



forms are prevailingly grasses, and their cultivation is the 

 purpose of the artificial pasture, but the meadow tendency 

 is shown by the coming in of perennial weeds. The inva- 

 sion of pastures by weeds suggests many interesting ques- 

 tions. Are the weeds natives or foreigners ? Are they 

 annuals or perennials ? What is the relative success of the 

 different invaders, and why are some more successful than 

 others ? A study of pastures will also reveal the fact that 

 there is great difference in the vegetation of mowed and 

 grazed pastures. The same effects are noted when natural 

 meadows are used for grazing. 



B. Woody societies. 



These societies include the various shrub and tree as- 

 sociations of mesophyte areas, associations entirely distinct 

 from the grass and herb societies. 



166. Thickets. — The mesophyte thickets are not so abun- 

 dant or impenetrable as the xerophyte thickets. They seem 

 to be developed where the conditions are not quite favor- 

 able for forests. An illustration of this fact may be ob- 

 tained 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 which 

 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 societies 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. In tem- 

 perate regions willow thickets are confined to stream banks 

 and boggy places, the plants evidently needing moist and 

 cool soil. Although the willow may be regarded as the 

 characteristic mesophyte thicket plant, there are other 



