538 Watson. — Sphagna: Habitats , Adaptations , and Associates. 
salt-marsh associations. S'. subnitens grows sparingly on the dunes near 
Southport; S. imbricatum is said to prefer wet heaths and bogs near the 
coast, but the author has found it more common in inland regions, in one 
case preserved in peat cuttings on the Pennines in a district where the living 
plant is now extinct. Experimental work with culture solutions has shown 
that the effect of high concentration of salts is variable according to the salt 
and the species employed, and that a concentration useful to most plants 
generally acts injuriously. 
Protective Devices. 
Sphagnum is often regarded as a pronounced hydrophyte, but any 
bryologist who carefully studies its structure and compares it with that of 
other Mosses must be struck by the presence of a number of characters 
which in other plants would be considered as of a xerophytic nature, 
enabling the plant to endure periods of drought. Such characters as com¬ 
pactness of habit, imbrication of the leaves, concave leaves with hooded or 
inrolled apical portions, formation of capillary chambers along which water 
passes, papillosity of the cell-membrane, intermixture of dead empty cells 
with living chlorophyllous ones, and presence of reservoirs for the storage of 
water, are usually accounted xerophytic devices in other Mosses, and all 
these characters are shown by species of Sphagnum . 
Imbrication of the branch-leaves to a greater or less extent is of general 
occurrence in the Sphagna, being entirely absent only in species which are 
usually found submerged (e. g. 5 . cuspidatum , Fig. 4, A) or in shaded moist 
situations (e. g. S. squarrosum ). In some plants of drier moorlands, e. g. .S. 
compaction (Fig. 5, w), 5 . cymbifolium var. conge stum , 5 . papillosum var. 
confer turn, the imbrication is very pronounced ; in fact it is a general rule 
that the drier the habitat the closer is the imbrication. S', subnitens 
(Fig. 2, a) in other respects has a similar structure to S', quinquefarium , but 
the latter usually grows in drier places, and has its leaves more closely 
imbricated. 
The branch-leaves of nearly all species are more or less concave, but 
generally the most pronounced hydrophytic member of a group of allied 
species has the leaves least concave. In the Cuspidata group, S. cuspidatum 
has a much flatter leaf (Fig. 4, b) and a wetter habitat than 5 . pulchrum 
(Fig. 4, F) ; similarly in the Subsecunda section, S', obesum (Fig. 5, R) may 
be compared with S. sub secundum (Fig. 3, K). The leaves, it may be noted, 
are usually larger or longer in the more aquatic species. 
Inrolling of the branch-leaf is another character which is general for the 
Sphagna. The lateral margins of the upper half of the leaf become incurved 
so that two small chambers are formed (Fig. 2, G), or the incurving may be 
so pronounced that a tube results (Fig. 4, F). In some Sphagna the 
inrolling is little shown, but the leaf is very concave and is cucullate, i. e. its 
