Jennings : A Botanical Survey of Presque Isle. 349 



rounding sand-plain, but the general advance of the shore-line lakeward 

 leaves the lagoon farther and farther inland, and, at the same time, 

 the vegetation increases immediately around the lagoon, so that de- 

 creasing amounts of sand will be drifted into the lagoon and into the 

 innermost zones of vegetation. From this it follows, that, after the 

 Popuhcs-Salix formation, each succeeding inner zone will have been 

 built up of less rapidly accumulated sand, and, as farther inland the 

 drifting sand is composed of finer particles, the zoned habitats are 

 thus characterized by successively finer-grained, more compact soils, 

 and in a general way each has taken a longer time in its building. 

 Furthermore, the accumulation of organic matter, humus, becomes 

 relatively a more important factor in the edaphic conditions of each 

 successive habitat. 



Contemporaneous with the initial stages of the /'^/?^Z?^j'-,5'rt'//a- forma- 

 tion there is a submerged formation in its initial stages in the waters 

 of the lagoon, and, as the ecological relations of the land and water 

 formations of the lagoons are very intimate, it has seemed best to take 

 up their consideration together. The different stages in the succes- 

 sion will be considered, as they are exemplified in the different lagoons 

 and ponds, in the order of development of the successive formations. 



Stage A. — Lagoon Aa (see Map). 



(rt) Potamogeton formation, 

 {F) Populus-Salix formation. 



The Potamogeton Formation. 



During the first few years of the existence of the lagoon, and con- 

 temporaneously with the beginning of the Popiilus-Smilax formation, 

 the lagoon is in many respects merely a portion of the lake, cut off by 

 a sand-bar, and of the same character as the lake itself ; but, consid- 

 ered as a habitat, the two are quite distinct. The waters of the lagoon, 

 not being mingled with the uniformly cool currents of the lake, but 

 being comparatively shallow and stationary, are subjected to greater 

 variations in temperature than are the waters of the lake. During the 

 growing season the water of the lagoon presents an excess of heat above 

 the waters of the lake ; during hot midsummer days temperatures above 

 90° Fahrenheit were noted in some of the more open lagoons south of 

 the Fog Whistle, while at the same time the lake along the beach-line 

 had a temperature of about 70° Fahr. 



