11 



naturalization. Such trees as the poplars, willows, Ailanthus, etc., 

 having been more or less extensively cultivated, have escaped and be- 

 come widely and thoroughly naturalized. Still other exotics, such as 

 the Paulownia, Paper Mulberry, Tallow- tree, etc., of less extensive 

 adaptability to varying clim .tic conditions, have become naturalized 

 in certain restricted regions. There are also many indigenous trees, 

 such as locusts and catalpas, which, through wide cultivation, have 

 largely increased the area of their distribution and become naturalized 

 outside of their original range. 



No systematic census of the actual range of such naturalized species 

 in North America has been attempted in the present catalogue, the 

 statements of their presence being often based upon more or less incom- 

 plete observations, but in all cases sufficient to warrant their introduc- 

 tion as associates of native species. Several cultivated exotic species, 

 not included in the present catalogue, show a tendency to become 

 naturalized. But the existence of such species seems for the most part 

 to be confined to old and neglected grouuds where, to a limited extent, 

 the trees have become self-propagated, though hardly capable of advanc- 

 ing and holding their own against the vicissitudes of more exposed 

 localities and other contestants for the soil. It is thought best, there- 

 fore, to exclude such trees till further observation proves them to be 

 naturalized. 



George B. Sudworth. 



