PLANT ASSOCIATIONS 



437 



in the formation of new plant communities in denuded areas or 

 in repopulating old ones where space still remains for the intro- 

 duction of new individuals or species. It is clear also that the 

 rapidity with which any given plant will invade such areas will 

 depend in part upon- the nature of the device which it possesses 

 for seed dissemination. The student is already familiar with some 



FIG. 283. Association of plants in a forest, blackberries forming a layer 

 associated with cedar (Thuja plicata), Cedar Mountain, Idaho 



After Clements 



of these devices in the plants previously studied in the field and 

 laboratory. Thus, the wide distribution of willows and poplars 

 along the borders of streams and on the shores of lakes or ponds 

 is due to the long, silky hairs on the seeds, which enable them 

 to migrate by means of air currents. The rapid migration and 

 wide distribution of such composites as the dandelion and yarrow 

 is due to the very effective parachute of hairs which serves as 

 a flying apparatus for the fruits of these plants. In the case of 

 fruits with wings, like the maples and pines, or of the heavy 



