180 BOTANICAL GAZETTE [MARCH 
slope less xerophytic than that of clay becomes more xerophytic 
as it becomes a river bluff, and conifers are found as well as 
deciduous xerophytes. As the bluff slopes become more gentle 
through erosive action, a mesophytic flora may gradually replace 
the xerophytes. 
Before the growing valley possesses a permanent stream 
there may be developed in the torrent bed a vegetation of 
amphibious shade plants, and when the water becomes more per- 
manent one may find algae and other hydrophytes. Spring 
brooks are infrequent, but they have a characteristic vegetation, 
due doubtless to the presence of more light and water. The 
development of a flood plain vegetation is well shown on river 
islands. First a sand bar develops, then an annual flora, and 
later a perennial vegetation in which Salix dominates. The river 
constantly erodes above and deposits below, hence the islands 
migrate down the stream, showing the oldest plant societies at 
the upper end. Depositing streams gradually develop a flood 
plain which shows an interesting succession of societies. Beyond 
the true hydrophytes there is commonly seen a Salix zone, then 
a zone of Populus and other trees on the older flood plain, and 
finally there develops a luxuriant mesophytic flood-plain forest 
which as a whole is permanent, though local retrogressions may 
occur. In some of these flood-plain forests there are found 
interesting southern types of trees. Occasionally meadows 
occur on flood plains in place of forests. Retrogressive proc- 
esses are active on flood plains, such as terrace formation, which 
is due to further erosion; terrace development tends to favor 
xerophytes. New channels are also cut off, leaving portions of 
the old river as oxbow lakes; here hydrophytes of undrained 
swamps come in, and one often sees trees of the old river margin 
together with shrubs and herbs of the undrained swamps. These 
latter phases, however, are ephemeral and the mesophytic flood 
plain as a whole increases constantly in area. 
The vegetation of undrained areas has a remarkably xero- 
phytic stamp; this is possibly due to the unfavorable oppor- 
tunity for root activity in undrained soils. In any event, these 
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