312 BOTANICAL GAZETTE [NOVEMBER 
decade. It was anticipated in 1910 that certain plants which 
require abundant humus would not be able to settle at once on its 
bowlders and gravel, nor could epiphytes soon find the necessagy 
trees to perch in. That the many mosses, ferns, and seed plants 
that grow all about the valley, not only in similar gravelly and stony 
soil along the trails, but even in the crevices of every rugged cliff 
and crag of the neighboring hills, should prove incapable of promptly 
and completely colonizing this valley was quite unexpected. 
The decisive causes responsible for this slowness of revegetation 
have not been determined. It may be remarked in the first place 
that browsing by animals is a negligible factor in the development 
_of the vegetation, since such animals are not allowed to run free in 
this region. Furthermore, it does not seem probable that the 
chemical nature of the rock can be the prime cause of this phe- 
nomenon. It is conceivable that at a later stage the soil formed 
by disintegration of the rock, which is an epidosite (or epidote 
gneiss), may determine the types of micro-organisms living in the 
soil and so the kinds of humus produced. The fact that a rather 
varied series of some thirty species of plants have been able 
to establish themselves in this valley shows that the soil, which is 
probably of fairly uniform chemical character throughout, is not 
especially unfavorable to plants. The distribution of the plants 
now growing in the valley seems rather to be related to the physical 
character of the soil. Plants are found growing where finer soil par- 
ticles have accumulated. Probably the most important hindrance 
to the increase of the vegetation is instability of the soil, which, 
in most areas of this rather steeply sloping valley, is being con- 
stantly changed, by erosion at some points and by deposit at others. 
It seems clear that in the future development of the plant cover- 
ing of this valley the existing vegetation after a time will establish 
more fixed conditions in areas now occupied. This will give the 
mycorhizal fungi and soil bacteria, which cannot thrive in this 
sterile gravel, a sufficient amount of vegetable matter on which to 
feed. There will then probably be a decided acceleration both in 
the spread of the plant species now present, and in the introduction 
of new species. The writer hopes, during the coming decade, to be 
able to study further and to report on the progress of the revegeta- 
tion of this valley. 
Jouns Hopkins UNIVERSITY 
BAL E, 
