110 



The walls are almost vertical. No plants exist, except those hanging 

 from the top of the cliff. In most successions on bare rock, lichens are 

 the pioneer plants, being found in the most xerophytic situations. No 

 lichens are found anywhere on the Hudson River limestone, on account, 

 no doubt, of the unstable nature of the substratum. This plantless stage 

 persists until after active undercutting by the stream has ceased. 



Then the cliff becomes less steep. The talus accumulates undisturbed 

 by the stream, and bears a considerable vegetation. In this stage occur 

 the pioneer cliff plants, occupying the shelves formed by projecting lay- 

 ers of limestone. The most of the plants are annuals and many of them 

 are plants that have slipped down from the top of the cliff. The follow- 

 ing plants are common in this pioneer association: Ambrosia arte- 

 misiaefolia, Poa compressa, Lactuca tcariola, Nepeta cataria, Melilotus 

 alba, Dipsacus sylvestris, Aster spp. 



The shale layers of the cliff change readily to soil, which is washed 

 down by rains. Layers of limestone thus left projecting break off of 

 their own weight and fall. With the consequent reduction in slope, an 

 increasingly lai'ger number of plants gain a foothold. In addition to 

 some of the pioneer plants mentioned above occur the following: Equi- 

 setum arvense. Aster nova-angliae, Daucus carota, Heracleum lanatum, 

 Melilotus officinalis, Verbascum thapsus, Elymus canadensis, Cornus 

 paniculata. 



Up to this point, the succession has been controlled almost entirely 

 by physiogenic factors. The stage in succession depends upon the slope 

 of the cliff. When, however, the slope has become sufficiently gentle to 

 permit the accumulation of a layer of soil, biogenic factors, those due to 

 other organisms, come in. The plants, particularly the grasses, hold the 

 soil and retard the further degradation of the cliff. The slope of a por- 

 tion of the cliff occupied by a mesophytic forest is about the same as that 

 of a portion occupied by the bush stage. Each plant association prepares 

 the way for the succeeding one by holding the soil, accumulating humus 

 and providing shade. 



The herbs are soon partially displaced by a bush association. The 

 most common species is Rhus aromatica, which often forms large col- 

 onies. Cornus paniculata, Salix longifolia, Rhus toxicodendron, Vitus 

 vulpina, Crataegus, Psedera, Ptelea trifoliata Rubus, Ribes and others 

 are accompanying species. 



