INTRODUCTION. 



flora is the southern limit for many northern species, while it is the 

 northern limit for fewer southern species, then some great northern 

 basin or plateau has contributed, presumably, more to its population 

 than the southern mountain or plain. Such a conclusion must be 

 checked by reference to the affinities of the characteristic types present ; 

 nevertheless, concerning the origin of a flora, we consider the test of 

 limit-species a reasonable one, and indeed, from experience, the most 

 critical single test we can apply. 



GENERAL RELATIONSHIPS. 



CHARACTERISTIC TYPES. We will now endeavor to make an appli- 

 cation of these canons to our Lackawanna, Wyoming and Pocono Flora. 

 The plants common in this region, but common also throughout the 

 Appalachian region, such as the Rhododendron, the Chestnut-Oak, the 

 Birches, the Witch Hazel, the sweet-scented fern, Dicksonia, we pass by as 

 plants which can tell us nothing we do not know in respect to the affin- 

 ities of a small flora. There are, however, certain species characteristic 

 of the hillsides lining this valley, frequently appearing in large numbers, 

 but which are not of universal occurrence in the Appalachian region. 

 Such are the Wild Indigo, the Sheep Laurel, the Sweet-Fern (Myrica 

 asplenifolia})\hz Sumach (Rhus copallina] ; and in certain places the 

 Purple Hardhack (Spinca tomentosa] and Andromeda liguslrina are 

 equally abundant. All of these are wanting or comparatively rare im- 

 mediately outside the valley to the north or west. 



The above plants and others we might name are characteristic of, 

 though not confined to, the drier region of the Atlantic slope, and their 

 presence here is due, no doubt, largely to the character of the soil. 



RARE PLANTS. Of the rare plants of the Flora exclusive of the Po- 

 cono, we might mention Linum sulcatum, Silene Pennsylvania, Phy- 

 sostegia, Pedicularis lanceolata, Cunila, Polentilla arguta, A Ilium cer- 

 nuum, Utricularia intermedia, U. cormita, and others, reported to be 

 scarce in Pennsylvania, while the pretty Potentilla tridentafa, occurring 

 on the summits of Penobscot Knob and Bald mountain, is known no- 

 where else in the State. In passing to the Pocono this list would be 

 greatly increased. Of these the only one we wish to speak of in this 

 connection is the Little Mistletoe {Arceutkobium pusillum) parasitic 

 on the limbs of the Black Spruce, discovered by the writer in 1886, and 

 noted in his "Preliminary List" of this region. It is known in a few 

 places in New York and New Hampshire, and nowhere south of our 

 stations, which are the only ones yet discovered in Pennsylvania. 



