242 



PLANT RELATIONS. 



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 



Fig. 207. A juniper heath interspersed with pastures. The growths of juniper are 

 very dense, excluding all other vegetation, and the grass or pasture areas are too 

 dry to form real meadows. — After Cowles. 



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 associations. 



These associations include the various shrub and tree 

 assemblages of mesophyte areas, assemblages entirely dis- 

 tinct from the grass and herb associations. 



