1913] COOPER—ISLE ROYALE 123 
is ordinarily a tree, the arbor vitae (Thuja occidentalis L.), which 
frequently sprawls over the shore rocks after the manner of a 
prostrate vine. : 
Finally the trees also appear as crevice plants, and not always 
last in time, for young seedlings and even aged but stunted indi- 
viduals are often found inhabiting the same crevice with the earliest 
herbaceous pioneers. Sometimes the largest cracks are lined with 
fair-sized trees while the rock surfaces between them are inhabited 
only by foliose lichens and cladonias. Any tree species may be a 
crevice pioneer, but Betula alba var. papyrifera occurs most fre- 
quently, with Pyrus americana, Thuja occidentalis, and Abies 
balsamea next in abundance. Growth in early years is sometimes 
of average rapidity, but the severe conditions soon make them- 
selves felt, and those trees that survive increase in size very slowly. 
A white spruce 1m. high and 8.75 cm. thick was found to be 87 
years old, and a balsam of the same height but only 7.5 cm. thick 
showed an age of 123 years. 
The preeminent importance of this subsuccession in the develop- 
ment of the forest should be emphasized. Through the soil and 
moisture-conserving capacity of the crevices and the resulting 
abundance and variety of their vegetation, and particularly because 
of the presence of creeping mat-forming shrubs, the establishment 
of the climax forest is very much hastened. 
3. The rock pool subsuccession—Much less important than the 
two preceding, but nevertheless contributing somewhat to the 
development of the forest by reason of the considerable amount of 
humus formed through its agency, is the rock pool subsuccession. 
Depressions of all shapes and sizes are common upon the rock 
shores (fig. 20). When they are of such a form as to contain 
standing water, the establishment of vegetation in them is quite 
different from that displayed in the crevices or upon the rock sur- 
faces. Rain and waves furnish the water, which is frequently 
only temporarily present. The subsuccession is thus hydrarch in 
general but its development is subject to interruptions due to 
partial or entire desiccation. In the smaller depressions, which 
usually contain water only part of the time, vegetation when 
present is of the crevice type. In the larger basins which contain 
