130 HARSHBERGER : PHYTO-GEOGRAPHIC SKETCH 
pentine)* are replaced by mesophytes, which thrive in rich alluvial 
soils ; mesophytes, by the wearing away of the soil and the for- 
mation of cliffs, by xerophytes, such as exist on the serpentine 
barrens of southeast Pennsylvania ; hydrophytes replace meso- 
phytes, when an area becomes too wet for the tenancy of ordi- 
nary plants. Thus, if we apply such principles to the study of 
our region, southeastern Pennsylvania, the dry ground formed by 
the post-Cretaceous uplift was the seat of a dense mesophytic 
forest, by erosion mesophytes of the plain were replaced by xero- 
phytes of the hillside, and by a still more pronounced base-level- 
ing process, the xerophytes were again replaced by mesophytes. 
‘When the submergence occurred and the forest was drowned, a 
‘series of hydrophytes filled the tidal estuaries to be replaced by 
terrestial plants at the next uplift. 
Shull + gives an interesting case in point. The distribution of 
Tsoetes saccharata Engelm. along Chesapeake bay is explained by 
the geomorphic movements of the coastal plain. The present 
elevation of land is such that the water of the bay is fresh about 
ten kilometers below the mouth of the Susquehanna river. Dur- 
ing periods of greater elevation the water was fresh farther to the 
south. When the land was so elevated that the water was fresh 
at the mouth of the Potomac river, favorable habitats along the 
shore of the bay must have been occupied by the progenitors of 
the colonies of /scetes saccharata which now occur in the upper 
estuarine portion of the tributary rivers. As the land sank and 
the rivers were ponded farther and farther from their mouths, new 
areas became adapted to the growth of /svefes, and new colonies 
were formed. Simultaneously the colonies furthest down stream 
were destroyed by the advance of salt water. In this way there 
came to be, instead of a single colony or group of colonies at the 
head of Chesapeake bay, as many distinct colonies as there were 
ponded tributaries. So long as the land continued to sink, the 
successful reproduction was on the up-stream side, and destruction 
followed pari-passu on the down-stream side until the present con- 
* Harshberger, J. W. Ray of the serpentine barrens of southeast Pennsylvania. 
Science, II. 18: 339-343. go 
+ Shull, G. H. So ned distribution of /soctes saccharata: Bot. Gaz. 36: 199- 
S 1903. 
