187] FLORA OF COLUMBIA AND VICINITY 45 



The sylva thus shows an alternation of plant elements. 

 Ravine plants, slope plants, hill plants, plain plants often occur 

 in close juxtaposition. Often a slight depression has but a 

 single ravine species, or a low ledge but one or two rupestrine 

 forms. There is, then, a subtle change of floral elements oc- 

 casioned by delicate changes in soil, elevation, and light, which 

 makes exact analysis impossible. 



i. Sylvales quercoides carpininae. The blue beech 

 subassociation of sylvan ravine plants occupies the deeper por- 

 tions of the ravines. There is usually a watercourse, at ordinary 

 times, however, dry. If the ravine has steep rocky banks with a 

 stream fed by an unfailing spring, the flora is that of the fon- 

 tinal rupestrine type. The blue beech subassociation is a re- 

 duced form of the riparian persisting amid mesophytic vegeta- 

 tion. The ligneous element is as follows: 



Juglans cinerea Platanus occidentalis 



Carya amara Physocarpus intermedins 



Carpinus Caroliniana Cercis Canadensis 



Ostrya Virginica Rhus Toxicodendron 



Quercus alba Euonymus atropurpureus 



Q. macrocarpa Celastrus scandens 



Q. acuminata Acer dasycarpum 



Q. rubra A. nigrum 



Ulmus fulva Aesculus glabra 



U. Americana A. lutea 



Celtis occidentalis Vitis riparia 



Morus rubra Quinaria quinquefolia 



Asimina triloba Tilia Americana 



Next to the watercourse such herbs as Collinsia verna, 

 Impatieiis spp., Circaea Lutetiana, Enpatorium ageratoides, and 

 Asplenium pycnocarpon occur, while on lower slopes Hydro- 



