136 



Excluding the two species of oaks, the ridges have 29 and 31 species, 

 respectively, of which only ten are common to both. This gives a 

 community coefficient (Jaccard ig02: 351) of 0.200, indicating at 

 once the great floral dissimilarity. There are, on the other hand, 

 thousands of acres still occupied by the black oak association, with 

 as yet no indication of the approaching succession. 



Rhus canadensis, var. illinocnsis, Tephrosia inrginiana, and Opun- 

 tia Raiinesqnii are the chief species concerned in accumulating fallen 

 leaves for conversion into leaf -mold. With the simultaneous de- 

 crease in light, the succession begins, and occasional plants of Poly- 

 gonahtni conimiitatum and Silene stcllata appear as pioneers. Agri- 

 monia mollis comes in somewhat later, and young plants of three 

 lianes appear. These are Vitis vulpina, Rhus Toxicodendron, and 

 Psedera qiiinqne folia. The last is especially common and valuable as 

 a succession index. Its long, slender stems trail for several feet along 

 the g-round, unless by chance they encounter a tree trunk to climb. 

 Rhus Toxicodendron seldom trails, but usually grows directly at the 

 base of some tree. Following these six species, which are easily rec- 

 ognized as pioneers, a number of others appear in irregular order. 

 The arborescent flora remains essentially the same, except for occa- 

 sional trees of Celtis occidentalis or Prnnus serotina, or, near the 

 river, Qucrcus rubra, Querciis niacrocarpa, Juglans nigra, Ulmiis 

 americana, Morus rubra, and Gymnocladus dioica. The undergrowth 

 is frequently dense, with numerous thickets of shiiibs, and the her- 

 baceous growth is tall and luxuriant. The following additional species 

 are especially characteristic : 



Asparagus officinalis Sanicula canadensis 



Smilacina racemosa Cornus Bailcyi 



Smilax herbacea Asclepias phytolaccoides 



Dioscorca villosa Lappida virginiana 



Anemone virginiana Scrophularia- Icporclla 



Ribes gracile Galium concinnum 



Celastrus scandens Bupatorium pnrpurenm 



Oenothera biennis Bupatorium urticaefolium 



It will be noted that a majority of these species have unusually 

 efficient means of seed dispersal. When an association develops de 

 novo some distance from the nearest existing area of it, the most 

 mobile species may naturally be expected to appear first, while the less 

 mobile species follow after greater intervals of time. The species in the 

 preceding list accordingly represent the mobile pioneers of an asso- 

 ciation, the usual dominant species of which have as yet not ap- 



