124 BOTANICAL GAZETTE [FEBRUARY 
pools that are nearly or quite permanent, the development is 
truly hydrarch; and zonation like that found in bogs, but of a 
simpler type, is usual. On account of the shallowness of the water 
the bog mat is seldom floating. By far the most important plant 
in the subsuccession is Scirpus caespitosus L., which starting its 
growth at the water’s edge gradually fills the pools with its dense 
hard stools. Frequently by growing across the high water outlet 
it raises the level considerably. Other species are occasionally 
more important in filling the pools. For instance, one depression 
was found to be partially filled by a mat composed of Polytrichum 
Fic. 20.—Rock pool upon one of the islands bounding Rock Harbor on the south- 
east: ees makes at the right; Andromeda and Alnus crispa at the left. 
commune L. and Aulacomnium palustre (L.) Schwaegrt.; bound 
together by the roots and rhizomes of plants growing upon it, 
among them Potentilla tridentata, Iris versicolor, Vaccinium pennsyl- 
vanicum. Another pool was being invaded by a mass of Climacium 
americanum Brid. Upon the turf mat habitually lives the interest- 
ing group of plants which has already been listed as occurring in 
moist crevices. Bog plants are frequent, especially Drosera 
rotundifolia L. and Polytrichum strictum Banks; and numerous 
crevice plants occur. Following, usually, the building of the turf 
mat come certain shrubs, the most important being Vaccinium 
