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occurrence of this tree, just as the mature hydrophytic flood-plains are 
indicated by Salix nigra. 
Toward the downstream side of these areas, the horizontal succession 
proceeds rapidly toward mesophytie 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 artemisizfolia 
Ambrosia trifida Helianthus strumosus 
Bidens frondosa Jupatorium 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. (8) 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- 
sion is toward a mesophytic forest of the beech-maple type. 
