Jennings: A Botanical Survey of Presque Isle. 315 



exact repetition of large parts of the Presque Isle sand-plain, especially 

 towards the southern part. 



The ecological conditions on the sand-plain are in some respects 

 somewhat less severe than on the drift-beach. One great difference 

 lies in the freedom of the sand-plain from wave-action ; a condition 

 which permits the establishment of biennials or perennials, which 

 perhaps might otherwise be able to endure the conditions obtaining 

 in the drift-beach. Further the sand-plain, being somewhat removed 

 from the lake, and in places being somewhat sheltered by dunes and 

 ridges, is more or less protected from the force of the wind, and on 

 the whole is probably less xerophytic. 



The soil of the sand-plain becomes with age more compact by 

 settling and by the sifting in and washing down by rain of the finer 

 particles of sand. At the same time humus is gradually accumulating 

 and so the general tendency of the soil is towards a greater capillary 

 capacity for water and a larger su])ply of available plant-food. Wher- 

 ever, because of certain conditions of wind- or wave-action, there has 

 been an accumulation of coarser sand or gravel in the surface layers 

 of the soil, the accumulation of humus and the filling up of the soil 

 with finer particles will take a much longer period, and during this 

 period more xerophytic conditions will prevail. 



The soil of that portion of the sand-plain included in the present 

 discussion is apparently, as a whole, of a considerably coarser texture 

 than is the soil of that part of the plain lying to the south of the 

 Key Post. This condition results quite naturally from the physio- 

 graphic mode of formation of the peninsula ; the direction of the drift 

 of the beach-debris being from the southwest and the coarser particles 

 traveling more slowly, the result is that in rounding the end of the 

 peninsula the coarser particles are left behind and so have contributed 

 to the growth of the more northern portion, while the finer material has 

 traveled farther and has contributed to the growth of the shore farther 

 to the southeast. 



The water of the coarser sandy or gravelly soil is more largely 

 gravitational and escapes quickly by percolation, following which the 

 air under the pressure of the strong winds circulates freely through the 

 larger air spaces, thus bringing about a somewhat greater evaporation 

 of the scant capillary water and also a rapid direct oxidation (^erema- 

 causis) of the little organic matter which may have accumulated. The 

 circulation of the air of the soil and also, indirectly, the denitrification 



