Jennings : A Botanical Survey of Presque Isle. 325 



salts. Physically the temperatures prevailing both in the soil and in 

 the air above the soil are more uniform, the aeration is much less, and 

 the light is very much weaker in the Pinus Strobus formation. 



The Quercus velutina Forest Formation. 



The gradual invasion of the pine forest by the black oak {Quercus 

 velutina) and finally the dying off of the pines, as they approach old 

 age, eventually results in a forest characterized by one facies, the black 

 oak, and hence to be called the Quercus velutina forest formation 

 (see Plates XXV and XXXIII). The black oak constitutes usually 

 from 85 to 95 per cent, of the primary layer in this forest and asso- 

 ciated with it are scattering individuals of the following species : 



Pinus Strobus, Tilia americana, 



Quercus rubra, Acer saccharinum, 



Quercus palustris, Sassafras Sassafras, 



Quercus borealis, Tsuga canadensis. 



The abrupt change in the character of the foliage canopy from the 

 dense, dark, evergreen pine woods to the light, deciduous, black oak 

 forest makes a very marked difference in the conditions of the habitat 

 with reference to insolation in the layers below the facies of the for- 

 mation, and the response to the changed environment is correspond- 

 ingly very plainly evident. 



Below the primary layer the four layers following are to be dis- 

 tinguished : 



The Secondary Layer. — The secondary layer is composed of small 

 trees and larger shrubs of varying sizes, consisting essentially of 

 smaller individuals of the species represented in the primary layer. 

 It is noteworthy in this connection, however, that there are relatively 

 more white pine saplings in this formation than in the Pinus Strobus 

 formation. The light conditions more nearly approximate to those 

 of the heath formation than to those of the pine forest. The result is 

 to a limited extent indicative of a possible alternation of formations. 



The Tertiary Layer. — The tertiary layer consists of small shrubs 

 and bushes, which owing to the presence of much Sniilax become in 

 places a tangled mass very difficult to traverse. The species repre- 

 sented in this layer are as follows : 



Sniilax herbacea, Aralia raceniosa. 



