412 ZiJSSSOIfS WITS PLANTS 



ing of several or even many kinds of plants. In 

 others, he will find that one kind strongly pre- 

 dominates and gives character to the flora; and 

 this is especially true of roadside floras, where 

 migration is unimpeded. 



522. If he watch this society throughout the 

 season, he will observe that its physiognomy, or 

 appearance, changes from month to month ; and 

 this change is not always such as comes from 

 the greater age and maturity of the plants, but 

 it is often the result of a progression or rotation 

 in the kinds of plants. If the September physi- 

 ognomy is normally one of aster or goldenrod, 

 what are the May and July characters ? And are 

 the asters usually preceded by the same types of 

 plants year by year and in various localities, or 

 are they preceded by any early-flowering plant, in- 

 discriminately ? 



523. If any society of plants is under tension, 

 it must follow that the withdrawal or modification 

 of any environmental factor will be followed by 

 changes in the flora and, consequently, in the 

 physiognomy. Let the pupil observe how a drain 

 put through a swale affects the flora, or how the 

 felling of the timber affects the undergrowth. The 

 farmer knows that sheep keep down bushes, from 

 their fondness for browsing ; when sheep are re- 

 moved from a copse or thicket, notice how soon 



