336 Annals of the Carnegie Museum. 



Secondary Species. — 



Prunus virginiana, Almis incana, 



Fragaria virginiana, Juniperus virginiana, 



Acer saccharinum, Quercus velutina, 



Toxicodendron pubescens, Poa compressa, 



Myrica carolinensis, Smilax herbacea, 



Psedera quinquefolia, Amelanchier oblongifolia. 



The layering in the Prunus forest is rather indefinite, but below the 

 facies are the smaller trees and saplings of the two wild cherries {Pru- 

 nus serofifia and P. pennsylvanica') together with a considerable number 

 of the secondary species, constituting thus altogether a layer of vari- 

 able importance throughout the formation. Below this secondary layer 

 is a more definite layer of bushes, the tertiary layer, dominated by 

 Rubus allegheniensis and Rubies occidentalis and occurring throughout 

 the formation, but best developed in the more open spaces, and tending 

 to form a fringing zone about the edges of the forest. Clambering 

 about quite promiscuously is the Celastrus scandens and some Vitis 

 vulpina and Smilax herbacea, which, together with the clumps of dead 

 and dying Myrica, often forms a tangle very difficult to push through. 



The herbaceous layer is very poorly developed in this formation, 

 the tertiary layer so completely occupying the habitat. However, 

 Poa compressa occurs in the few openings among the bushes and is 

 accompanied sometimes by Fragaria virginiana. 



The successor to the Prunus forest-formation is evident from the 

 seedlings of Quercus vehiiina, which appear early in the life of the 

 formation, and which gradually become more important, until finally the 

 wild cherries are relegated to a minor position. Were it not for the 

 difficulty with which Quercus is disseminated, as compared with the 

 species of Prunus, the Myrica thicket-formation would, undoubtedly, 

 be succeeded directly by a Quercus forest. 



The Prunus forest- formation is evidently the homolog of the 

 Pinus Strobus forest-formation, in that it immediately precedes the 

 Quercus velutina forest, but, although the habitats are closely similar, 

 the two formations have very few species in common in the lower 

 layers, on account of the difference in the amount of light under the 

 forest canopies. 



