318 BOTANICAL GAZETTE [MAY 
white birch are the trees which have seeds adapted for rapid 
migrations, and are also rapid growers. Because they are per- 
ennial, and can thus get an early start each year after the first, 
they soon overtop the fireweeds and gradually shade them out. 
Ostrya Virginica, Prunus Pennsylvanica, P. serotina, and Quercus 
yubra are in some places associated with the poplars and birches. 



—A spring hillside with young growth of arbor vitae. ‘Ibe hilf in the 
ig oe been cleared of a maple-hemlock forest and is used for a pasture. 
In the absence of pine trees in the vicinity to furnish pine seeds, 
this poplar-birch society is maintained until deep shade condi- 
tions are again obtained for the growth of the maple-beech seed- 
lings. Then, just as in the normal development the pines are 
replaced by the maples and beeches, so in the clearing develop- 
ment the poplar and birches are likewise replaced. The pop- 
lars go first, then usually the birch and hornbeam, and finally 
the cherry trees drop out. 
If, however, pine trees in the vicinity of a clearing can fur- 
nish seeds, the pines occupy a place in the life history. The 
