1913] COOPER—ISLE ROYALE 125 
uliginosum L. Vaccinium pennsylvanicum, Andromeda glauco- 
phylla, and Alnus crispa also occur commonly. In some pools 
Andromeda is advancing directly into the water, the turf mat being 
absent. Calamagrostis canadensis (Michx.) Beauv. is usually a 
noteworthy companion to the shrubs. 
4. The heath mat.—We have now traced the subsuccessions as 
far as they remain distinct. In the coalescence of the three the 
species of the crevice series are the active agents. Such plants as 
Juniperus horizontalis, J. communis var. depressa, Arctostaphylos 
Uva-ursi, and the krummholz form of Thuja occidentalis send out their 
trailing stems in all directions from the crevices in which they are 
anchored, winding through and among the cladonias of the rock 
surfaces and the various vegetation of the low places that were 
formerly pools. The continued growth of these three elements 
produces a firm compact mat, strongly attached in the crevices and 
depressions. 
The history of an area may often be traced by an examination 
of the successive layers of the vegetation growing upon it. In one 
place four stages were discovered. Representing the first were 
scattered plants of Potentilla tridentata and Deschampsia caespitosa 
growing up through the mat and traceable to crevices in the rock 
beneath. Second in order was a thick layer of Arctostaphylos 
spreading over the rock in all directions but rooting in the crevices. 
The third stage was represented by Juniperus horizontalis growing 
over the Arctostaphylos; and the fourth by a few plants of the climax 
forest, among them Aralia nudicaulis and Maianthemum canadense. 
The order observed here is by no means universal; in fact, Junip- 
erus horizontalis usually precedes Arctostaphylos. 
The four important creeping shrubs differ greatly in their 
effectiveness in mat formation. In the case of Thuja the growth 
is too open’and the branches reach too great an elevation above the 
surface to favor the accumulation of humus; and the same to a 
less degree is true of Juniperus communis var. depressa. Juniperus 
horizontalis by reason of its closely appressed habit is much superior. 
Arctostaphylos is probably the best of all, since its overlapping 
leaves almost entirely prevent the washing away of waste from 
beneath itself and other plants with which it grows. It is also 
