HARPER: PLANT POPULATION OF MICHIGAN a7 
An interesting feature of the precipitation is that late summer is the 
wettest season, by a small margin. In this particular our region 
resembles the pine-barren portions of the coastal plain, and differs 
notably from southern Michigan and the whole Mississippi valley, 
where spring and early summer are the rainy seasons. Coming as 
it does just when the evaporating power of the sun tends to be 
greatest, the late summer rain helps keep the ground-water level 
constant and favors the formation of peat. It may bea mere coin- 
cidence that two such sandy and peaty and “piney” and thinly 
settled regions as northern Michigan and peninsular Florida should 
both have a late summer rainy season, but it is more than likely 
that the rainfall has influenced the soil somewhat, or even vice 
versa through the vegetation, for there is no apparent physiographic 
reason why the seasonal distribution of rain should be any different 
in Michigan from what it is in Ohio and Indiana, for example. 
Vegetation.—The aspects of the vegetation or the composition 
of the plant associations have been described at considerable 
length in some of the papers cited, particularly those by Livingston 
and Gates (and Gates’s are illustrated), so that it is not necessary 
to say much more on the subject here, except for pointing out 
some fundamental principles often overlooked, and giving quanti- 
tative data, which have not been supplied before except for very 
small areas or for only a few species. 
The dry uplands have vegetation of three principal types, 
correlated with soil differences. On the more clayey soils the 
original forests evidently were mainly of hardwoods and hemlock, 
making a dense shade and considerable humus. On the most 
sterile sands forests of jack pine prevail,* while in intermediate 
habitats, covering most of the upland area, white and red pines 
seem to have been the dominant trees before the lumbermen ap- 
peared on the scene. 
The streams have more or less meadow and river-bank vege- 
tation along them, but few species seem to be confined to such 
situations in the region under consideration. Flat areas adjacent 
* See Spalding’s paper on “The Plains’’ previously referred to, and Beal’s Michi- 
gan Flora, pp. 16-18. The jack pine, Pinus Banksiana, resembles the Florida spruce 
pine, P. clausa, very much in gen appearance, habitat, and relations to fire. 
(See Ann. Rep. Fla. Geol. Surv. 7: 142-144, 155- 1915.) 
