rgt1] DACHNOWSKI—CRANBERRY ISLAND 29 
THE CENTRAL ZONE 
This zone is situated centrally on the island. It occupies the 
larger part of the area of the island, and in its floral structure 
is very distinct. The plants consist principally of Vaccinium 
(Oxycoccus) macrocarpon and several species of Sphagnum, with 
Rhynchospora alba, Eleocharis obtusa, Aspidium Thelypteris, Duli- 
chium arundinaceum, Carex comosa, Scheuchzeria palustris, Juncus 
canadensis, Eriophorum virginicum, Osmunda cinnamomea, Drosera 
rotundifolia, Menyanthes trifoliata, several orchids, and other 
light-demanding forms variously grouped. The surface is char- 
acterized by hollows and elevations. The latter are due, in the 
opinion of the writer, to various causes; in part to the upward 
growth of sphagnum competing with cranberry, in other places 
because of a mutual protection which is afforded by the massing 
of forms of a similar height against excessive loss of water. In 
still other places, cranberry and sphagnum are growing beneath 
shade-producing forms, notably around ferns and invading maples 
and sumachs. Here they possess the ability to grow up in a 
manner giving rise to a thick soft mass, raised to a considerable 
height, more at the center than at the periphery. The maximum 
height to which cushions of sphagnum can grow is limited by the 
vertical saturation gradient of the water content in the air. The 
vertical level of this vegetation is otherwise fairly uniform, and 
varies only between 6 inches (15 cm.) and 1.5 feet (45 cm.) above 
the peat substratum, forming a low, dense, compact growth. The 
taller growth of grasses and sedges and the occasional bushes of 
Gaylussacia baccata, Prunus melanocarpa, and P. arbutifolia occur 
chiefly scattered and as open facies. They do not dominate the 
general vegetation enough to interfere with the transpiring organs 
of the plants at the lower level. 
A more detailed study of the distribution of the species in the 
lower stratum shows habits of growth giving rise to vertical layers 
sufficiently defined to recognize vertical zonation; especially the 
differences of growth in height in the sphagnums, Gaylussacia, 
and Vaccinium in areas of varying physiological aridity show that 
the plants are adapted to a given average supply of water. But 
