243 



occurrence of this tree, jiif^t as the mature hydrophytic flood-plains are 

 indicated by Salix nigra. 



Toward the downstream side of these ai-eas, the horizontal succession 

 proceeds rapidly toward mesophytic conditions. Soil accumulates around 

 the trees and the bare rocks and gravel are soon covered. In these loca- 

 tions a rich mesophytic vegetation is found. The following species were 

 noted in such a location : 



Salix nigra Ambrosia artemisisefolia 



Ambrosia trifida Helianthus strumosus 



Bidens frondosa Eupatorium perfoliatum 



It is to be understood that the term "xerophytic" is here used in its 

 broad sense, to indicate that the conditions of plant life are unfavorable 

 in these areas. The extreme thinness of the soil will render water absorp- 

 tion difficult, however plentiful it may be. The range of temperature 

 changes is larger than elsewhere. The trees and other plants are subject to 

 partial submergence at every rise of the river. Perhaps the greatest actual 

 injury comes from floating' ice in the winter floods. Sycamores on a xero- 

 phytic flood-plain near the bend of the river were more than half cut in two 

 by floating ice, and the upstream side of almost every trunk was dead. The 

 willows commonly show a distinct leaning in the direction of the flow 

 of the river. 



We may summarize the results of the investigation as follows : Five 

 distinct plant formations are recognized in the region studied, and each 

 plant association may be referred to one of these five formations: (1) In 

 the rock bluff formation, all stages of the succession from the bare, plant- 

 less cliff to a bluff covered by a mesophytic forest, are found within the 

 area under consideration. (2) The same stages of the succession occur 

 in the rock ravine formation. (3) A pond formation occurs at various 

 points of the stream, and the stages of the succession from this condition 

 towards mesophytism may be traced. (4) Definite hydrophytic flood-plains 

 show the usual succession towards mesophytism. (5) Flood-plains of a 

 xerophytic nature occur commonly. The succession to mesophytism in this 

 formation is very rapid. In all of the formations, the trend of the succeS' 

 f^jon is toward a mesophytic forest of th^^ beech-maple type, 



