138 College of Forestry 
thallus like spreading rosettes of lichen (Parmelia tilcacea)* 
(see Fig. 21). Another boulder (angular, with sloping top) 
had a mat of partly humified duff from one to three inches 
thick, knit together by the roots and rhizomes of polypody 
which made a close vegetation cover. Some larger plants 
(spermatophytes) were gaining a foothold here. Upon yet 
another and much larger boulder the vegetation history had 
progressed so far as to build up a soil mat one to six or more 
inches in depth which retained water so well that it supported 
not only raspberry canes but fair-sized saplings. The seep- 
age of water from this soil mat kept the sides of the boulder 
wet for at least a day or two following a soaking rain and an 
algal growth (probably Protococcus and blue-greens) cov- 
ered the vertical face while mats of moss had formed where- 
ever a slight fissure or ledge gave mechanical support. Of 
course some of you will have seen fair sized trees growing 
on top of large boulders in the Adirondacks. See also Fig. 
24. 
A study of conditions in the Long Lake West district seven 
years after the disastrous forest fire of 1908, reveals among 
other items to be mentioned subsequently that bare rock sur- 
faces are gradually but of course slowly in most cases, being 
covered by mats of moss — especially Polytrichum Fig. 48 
— which gains its foothold first at fissures and spreads from 
these. In some cases the rock surface lies so that seepage 
keeps it moist in which case the spread of moss is relatively 
rapid. On the other hand, all the gain of a wet season may 
be lost by drying up of the moss mat in very dry weather. 
It may be mentioned here that even in the severe fire of 1908, 
not all the blanket of vegetation debris—duff, peat, leaf mold 
— was burned off so that the return of vegetation is faster 
than on surfaces that were burned bare. The accumulations 
of drier — often dustlike — duff are largely covered by a 
Polytrichum mat which is being invaded by red raspberry 
1In this connection, see Cooper, W. S., The Climax Forest of Isle 
Royale, Lake Superior and its Development. Bot. Gaz., 55:1913, p. 118 
et seq. Also Whitford, H. N., The Genetic Development of the Forests 
of Northern Michigan. Bot. Gaz., 31:1901, p. 306 et seq. 
