1909.] NATURAL SCIENCES OF PHILADELPHIA. 399 



of the water varies within wider limits such plants are checked in their 

 growth or completely exterminated. This was emphasized in our 

 discussion of the salt marsh vegetation found in Upper Wreck Pond, 

 and it accounts for the limitation of fresh water species when the water 

 in which they grow reaches a certain degree of salinity. 



The study of the salt marshes, salt ponds and fresh water lakes of 

 northern coastal New Jersey has developed many features of phyto- 

 geographic importance. The origin of the lakes and ponds, it has been 

 proved, is due to the advance of sand dunes across the outlet of various 

 streams that have their rise some distance inland. Most of the larger 

 streams, that have kept open their outlets to the sea, are lined for 

 some distance from the ocean by salt marsh vegetation, which shows a 

 number of characteristic plants in pure association. As the sand bar 

 encroached upon the outlets of the streams the water became somewhat 

 brackish, and the conditions favorable to the salt marsh species became 

 more precarious, one species at a time disappearing with the decrease 

 of water saltiness. Finally, near the heads of several of the bays, and 

 also in several of the smaller ponds, the water becomes more and 

 more fresh, and we find a replacement of salt marsh plants by those 

 accustomed to grow in or near fresh water streams. Similarly salt 

 ponds have by a slow process of change been converted into fresh 

 water lakes, and the vegetation has changed accordingly with the 

 altered physiographic conditions. Instead of the vegetation of the 

 New Jersey coast remaining of a fixed type, we can trace, by a com- 

 parison of the several plant formations in various conditions of develop- 

 ment, the stages through which the vegetation has passed in reaching 

 its present aspect, and the age of these different associations of plants 

 can be determined by comparing the marsh areas given on the present 

 survey maps with those indicated on the older survey maps of the 

 inlets made at various times in the past by engineers for harbor 

 improvement. 16 The development has been a progressive one, and 

 all in a certain direction from an original salt marsh vegetation to a 

 fresh water and dry land flora. If at any time in this orderly sequence 

 the sea should break through the barrier of sand, or remove it by the 

 ordinary physiographic processes which are constantly active in the 

 region, then there is a reversal of the orderly progression of one type 



16 See for this purpose the illustrated paper by Prof. Lewis M. Haupt, on 

 "Changes along the New Jersey Coast," Annual Report State Geologist of New 

 Jersey, 1905: 27-95; 1907: 72-81, and for methods of using similar survey maps 

 see Otto E. Jennings, "A Botanical Survey of Presque Isle, Pa.," Annals of the 

 Carnegie Museum, V: 294-305 et seq. 



