157] FLORA OF COLUMBIA AND VICINITY 15 



Certain species seem normally to occur in places where they 

 never reach their perfection of symmetry and size. Thus the 

 red cedar (Juniperus Virginiana) is frequent on the extreme 

 edges of cliffs, where it is dwarfed, misshapen and often half- 

 dead, while the finest specimens of the species here, occur along 

 a small stream north. Doubtless upon the cliffs the red cedar 

 grows in the region of least tension, its competitors there being 

 few and principally herbs. 



Again the same species occasionally assumes various vege- 

 tation forms according to circumstances. Thus the poison ivy 

 (Rhus Toxicodendron) is a trailer or creeper on the ground, 

 when no support is found; and when the herbage begins to 

 grow rank, it often sends up branches which grow erect and 

 show no tendency to creep or climb. On cliffs it is a dinger, 

 mantling the ledges in the manner of the ivy. On trees it be- 

 comes a true vine and ascends to their summits with great ease. 

 These differences appear so marked that some authors have 

 based specific distinctions upon them. The Virginia creeper 

 (Quinaria qiiinquc folia) behaves similarly, except that it does 

 not send up erect shoots when trailing on the ground. The bit- 

 tersweet (Celastrns scandens), on the other hand, seeks always 

 a support, and if none offers, it twines around itself in large 

 rope-like strands. 



8. PLANT SOCIETIES OF THE REGION. 



The indigenous vegetation of the region falls into three 

 primary divisions or provinces ■} 1. The hydrophytic, consisting 



^The divisions are as follows: 



Primary division, or province, I. Hydrophytes. 

 Zone, or secondary division, A. Aquatiles. 

 Sub-zone, or plant formation, i. Natantes. 

 Association, or plant community, a. NympiiKoides. 

 Sub-association, i. Carpinin^. 

 Vegetation-forms, a. Arbores. 

 Time successions, a. Vernales. 



