Jennings : A Botanical Survey of Presque Isle. 321 



acting as a mulch. The temperature of the soil is much more uniform 

 than on the exposed sand-plain ; especially are the maximum tem- 

 peratures much reduced. The liberation of carbon-dioxide during the 

 process of humification also probably results, in connection with humic 

 acids, in the formation of a|)preciable amounts of carbonates, humates, 

 silicates, etc., which in solution in the capillary water of the soil are 

 available as plant-food. The formation of the various mineral plant- 

 foods in this manner is rendered more probable by the variety of 

 minerals represented in the sand, feldspar, hornblende, gneiss, mica, 

 magnetite, etc., derived mainly from the glacial till, which covers the 

 land along the shores of Lake Erie. 



There must be a considerable growth of fungal mycelium through- 

 out the layer of sandy loam, as the Geaster is quite abundant and well 

 distributed in the formation (see Plate XXXI). The presence of 

 Lupiniis with an abundance of root-nodules points to the fixation of 

 atmospheric nitrogen, and the bacteria, which must accompany the 

 humus, must finally indicate the formation of nitrates in the soil. 



To briefly recapitulate : the soil of the heath as compared with the 

 soil of the sand-plain is more stable ; is more uniform in temperature 

 and moisture ; is less excessively aerated ; has greater capillary and 

 hygroscopic capacity attended by less rapid leaching ; has an upper 

 layer of sandy loam, acting as a mulch ; eremacausis is very slight, if 

 present at all ; but humification and nitrification must occur, indirectly 

 resulting in the production of various salts available as plant-food. 



The Finns Strohiis Formation. 



As stated in the preceding discussion the mature heath has reacted 

 upon its ecological environment to a quite marked extent, and in so 

 doing it has at the same time brought about certain conditions suit- 

 able to other species, which will thus be able to eventually replace the 

 heath-formation. Jiiniperus virginiana and Arctostaphylos Uva-Ursi 

 can accomplish ecesis and thrive in dry sandy or gravelly soils, ^* or 

 on dry limestone hills or barren flats, in the case of Jtiniperus ; ^^^or 

 in dry sands and on exposed xerophytic mountain-tops, in the case 

 of Arctostaphylos. '"^ So it is, also, to a less extent, with Fifius Strobus. 



The white pine, throughout its range, whether on islands in a 



5* Britten, W. E. /. c. 



s^Mohr, Charles. "Notes on the Red Cedar." U. S. Dept. Agricul., Div. 

 Forestry, Bull. 31 : 28, 1901, aW Adams, C. C. /. c, pp. 24, 29, etc. 



