1913] COOPER—ISLE ROYALE 121 
luxuriant mat of vegetation may form upon a bare rock surface 
without the aid of crevice plants. The process, however, is 
exceedingly slow, and the mat thus formed is not so firmly attached 
and so surely permanent as that which is bound together by the 
trailing stems from the crevices. 
2. The crevice subsuccession.—Crevices due to bedding planes, 
joints, and to differential weathering occur more or less commonly 
in all the rocks of Isle Royale. Where they are abundant the 
stages of the rock shore succession are passed through rapidly; 
where they are rare the process moves slowly. The soil which forms 
Fic. 19.—Potentilla tridentata as a crevice plant; outer shore of Mott Island 
in the crevices through disintegration of the rock, or is washed 
into them from the surrounding surfaces and the forest above, or 
is carried to them by the wind, is held within them, at least in 
part. Water flowing into or through the crevices also is retained. 
They form natural collecting places for seeds carried by wind, birds, 
and surface wash. Thus it is not surprising that nearly every 
crevice, large or small, is peopled by plants of many kinds. The 
number of species found growing as crevice plants is very great, and 
includes forms ordinarily occurring in diverse habitats. There are 
listed in my field notes 100 species, or one-fifth of the recorded 
flora of Isle Royale, as growing in crevices upon rock shores, and 
this list includes such forest plants as Maianthemum canadense, 
