85 



This extension ol range seems to be clue to actual migration of 

 the species concerned, rather than to any space-\ariation in the 

 local environmental conditions. Most of the introduced species 

 are typically sun plants and never occur under the deep shade of 

 the aspen thickets. The annual growth of the thickets therefore 

 normall}' tends to reduce the opportunity for their development. 

 Since the aspen seeds germinate poorly except under the peculiar 

 conditions immediately following a fire, new thickets are seldom 

 formed and wide treeless expanses of Pteris aquilina occur every- 

 where in the association. The presence of such treeless areas 

 bears no relation to the penetration of introduced species, and 

 such areas may occur adjacent to the roadside and yet be vir- 

 tually devoid of all foreign plants. 



Immigration of introduced species certainly takes place every 

 year : ecesis only when and where local conditions are favorable. 

 The spring and early summer of 1916 and 1917 were both un- 

 usually cool and rainy. Under such conditions the surface 

 layers of sand keep moist for a longer time and afford a better 

 opportunity for seed germination. The extension of range shown 

 by the table may be accounted for by this climatic feature. 

 Even at the present time the number of individuals in the further 

 quadrats is small, while the great mass of plants occurs in the 

 first quadrat, adjacent to the wheel track. 



Although many of the species are perennial, their presence seems 

 to be frequently due to repeated immigration, rather than to 

 their own reproduction. Such hardy species as Achillea mille- 

 folium and Chrysanthemum Leucanthemum did not occur in 191 7 

 in stations where they had bloomed in 191 5, and their reappear- 

 ance awaits the coincidence of favorable environmental condi- 

 tions and new immigration. 



III. The Introduced Flora of Hardwood Clearings 



There are still some small areas of beech-maple hardwoods 

 being cut for lumber. All the supplies of hay and grain for the 

 lumber camps are hauled in by team, so that there is an excep- 

 tionally favorable opportunity for the immigration of seeds of 

 many foreign species. The chance of their germination and 



