Jennings : A Botanical Survey of Presque Isle. 329 



scribed, but it nevertheless differs from the latter so radically, that it 

 seems best to give it a distinct rank. 



This succession comprises a total area about equal to the succession 

 just described, but it is considerably broken up by ponds and lagoons, 

 so that it is scattered over a region much larger. Beginning near the 

 Key Post it extends to the east and southeast of the Beach-Sand Plain- 

 Heath-Forest Succession, reaching in the one direction to the U. S. 

 North Pier and in the other direction to Presque Isle Bay, extending 

 southwestward to the Chimney Ponds. 



The initial and final stages in the two successions are much more 

 similar than are the intermediate stages. As will be discussed farther 

 on, the intermediate stages represented by the heath and the white 

 pine forest are really formations derived from a distinct forest center, 

 and therefore representing altogether another succession than the 

 intermediate stages in the succession under discussion. The presence 

 of stages of another succession, belonging to a distinct forest center, 

 indicates some considerable difference in the ecological conditions 

 obtaining in the habitat, and this difference is probably that of dif- 

 ferent conditions of soil moisture. The amount of soil moisture 

 depends mainly upon the physical texture of the soil, particularly 

 the size of the soil particles, and also upon the amount of humus 

 present. The more northern and lakeward portion of the peninsula 

 is built up of the coarser deposits of the eastward running littoral 

 current, thus a coarser soil occurs here with less capillary moisture 

 and more xerophytic conditions. 



The amount of moisture in the soil being different in the two habitats, 

 there might be expected to occur a corresponding difference through- 

 out the entire successions, but in the initial stages of the successions 

 there is practically no humus in the habitat, and the exposure is in 

 both cases so severe that the corresponding formations are practically 

 identical. In the final stages, on the other hand, the humus has accu- 

 mulated in both habitats to such an extent as to bring about very 

 similar edaphic conditions, thus permitting the occupancy of the two 

 habitats by the same formations. It is only in the intermediate stages 

 of the successions, where the conditions of exposure and the content 

 of the humus differ considerably, that differences of corresponding 

 stages in the formations may occur. 



