Jennings : A Botanical Survey of Presque Isle. 319 



the position of heaths on the windward slopes of the dunes of Lake 

 Michigan, notes that : " The key to these facts is exposure to desiccat- 

 ing factors, especially heat, cold, and winds. . . . There is a vegeta- 

 tion carpet and a covering of humus. Both slopes have a inesophytic 

 soil ; the leeward slope has also a mesophytic air, but the windward 

 slope has a xerophytic air." The heath at Presque Isle is character- 

 ized, as is also the northwestern portion of the sand-plain, by strong 

 winds and extremes of heat and cold, but, as mentioned in the pre- 

 ceding paragraph, the conditions of the soil have become somewhat 

 more favorable to plant-life than in the sand-plain. 



The heath is a closed formation characterized by two facies — Jictii- 

 perus virginiana and Arctostaphylos Uva-Ursi (see Plate XXIX). 

 The formation has few secondary species and varies but little from 

 season to season in general appearance. However, in a few places the 

 Liipinus perennis Society becomes quite conspicuous in a June Aspect. 



Secondary Species. — 



Geaster hygrometricus, Andropogonfurcatus, 



Poa pratevsis, Lithospervium Gmelini, 



Pinus Strobus, Quercus velutina, 



Prunus serotma, Celastrus scandens, 



Toxicodendron pubescens, Rubiis alleghoiiensis, 

 Rubus occidentalis. 



Most of the species included in the above list are to be found most 

 abundantly in adjoining formations, where they more properly belong. 

 Some are to be classed as relicts of the preceding formation — Litho- 

 spermum and Audropogon ; others are invaders from the thicket forma- 

 tion immediately along the ridge to the east — Rubus, Rhus, Celastrus, 

 and Prunus; still others are invaders from* the forest formations — 

 Quercus velutina and Pinus Strobus. 



The bearberry (^Arctostaphylos') spreads quite rapidly in all direc- 

 tions over the sandy soil by means of its long prostrate vegetative 

 shoots, and although during the winter months it usually has an abun- 

 dance of bright colored berries eaten by birds, to which some of its 



Survey of the Huron River Valley, III." Bot. Gaz., \o: 275, October, 1905; 

 Whitford, H. N. "The Genetic Development of the Forests of Northern Michi- 

 gan." Bot. Gaz., 31 : 298-299, May, 1901 ; and A^&ms, C. C. "An Ecological 

 Survey in Northern Michigan." Rpt. Mich. State Bd. of Geol. Survey, 1905 : 24, 

 1906. 



