179 



Corniis florida, Ostrya virginiana, and Sassafras vanifolium. 

 These are sometimes so abundant as to produce a distinct 

 stratum of vegetation. Many of the shrubs and herbaceous 

 plants of the oak-hickory stage are still prominent, but their 

 number is augmented by numerous new arrivals, among which 

 are many pronounced shade plants. Three species comprise 

 the bulk of the shrubby undergrowth, viz., Kalmia latifolia, 

 Hamamelis virginiana, and Viburnum acerifolium. Some of the 

 characteristic herbaceous plants not heretofore noted are given 

 in the subjoined list. 



Adiantum pedatitm Corallorrhiza maculata 



Aspidiiim noveboracense ■ Epifagus virginiana 



Aspidium spinulosiim Epipaciis piibescens 



Botrychium virginianum Medeola virginiana 



Lycopodium lucidulum Monotropa uniflora 



Phegopteris polypodioides Polygonatiim biflorum 



Arisaema triphyllum Sanicula marilandica 



Aster divaricatus Solidago latifolia 



Brachyelytrum erectum Trientalis americana 



Collinsonia canadensis Trillium cernuum 

 Trillium erectum 



A series of changes in vegetation such as has been described in 

 connection with the succession on the trap ridges is actuated 

 almost entirely by biotic agencies and is therefore termed a 

 biotic succession. The general manner in which biotic agencies 

 institute succession has been admirably summarized by Cowles,* 

 and some of his observations may be briefly stated here. All 

 external factors which affect the plant reside either in the soil 

 or in the air. Of the soil agencies humus is by far the most 

 important in influencing succession. Due to its great capacity 

 for water retention the accumulation of humus on uplands causes 

 an increase in soil moisture; while in depressions, for obvious 

 reasons, it has the opposite result. The change thus brought 

 about in the water content of a soil is without doubt the most 

 important effect of humus, and, in the opinion of Cowles, is 

 perhaps the most significant of all factors influencing succession. 



* 1911, op. cit. 



