The Problems of Plant Distribution 269 



etc., with a mixture in places of pines, firs, hem- 

 locks, and some other conifers. Its general char- 

 acter approximates that of northern Europe, and it 

 merges into the still more uniform forest flora of 

 the sub-polar zone to the North. But even in the 

 more northerly territory of the Eastern United 

 States, types occur which are quite absent from the 

 European flora. Hickories, walnuts, and sassafras 

 are extra-European genera which exist in Canada, 

 and somewhat further south other peculiar forms, 

 magnolias, tulip-trees, persimmons, gums, locusts, 

 and other less familiar types, absent from the Eu- 

 ropean forests, add variety to the magnificent forest 

 which reaches its finest development on the slopes 

 of the Southern Appalachian mountains. In the 

 southernmost parts, e.g., Southern Florida, a strong 

 tropical element derived from the neighboring West 

 Indian flora is conspicuous. This includes such 

 forms as the palms, mahogany, and wild pineapples. 

 Some tropical types have even made their way far 

 north. Thus the pawpaw, a member of the tropical 

 family of Custard-apples, occurs as far north as 

 Southern Michigan. Besides the trees, there are 

 many beautiful shrubs and herbs that characterize 

 these splendid forests. Before the leaves appear 

 in the spring, many delicate herbaceous plants, 

 blood-root, Dicentra, spring beauties, dog-tooth vio- 

 lets, anemones, trilliums, etc., rapidly spring up, ex- 

 pand their flowers, and as quickly disappear, to 

 rest until the next spring. Many flowering shrubs 



