544 Watson. — Sphagna: Habitats, Adaptations, and Associates. 
The orientation of the two kinds of cells, their relative sizes, shapes, and 
porosity, will be discussed in a more detailed manner later. 
Papillosity of the cell-wall (Fig. 3, p) is a character met with ill few 
species (S. papillosum, S. imbricatum). In 6'. medium , S'. squarrosum , 
S. teres , and .S. Wulfianum papillae are sometimes met with, but they are 
scarce or very minute. In some varieties of .S', papillosum , which are 
generally plants of wetter situations than the type, the papillae are absent 
(var. leve) or very minute and only met with in some of the leaves (var. 
subleve ). .S', imbricatum has longer papillae (Fig. 5, O), but has not a drier 
habitat; in some other respects (e. g. greater width of chlorophyllous cell in 
respect to the hyaline one) its structure is more hydrophytic. Its variety 
affine has its cell-walls devoid of excrescences and is found in more con¬ 
stantly wet places. 
Many examples of chambers for the storage of water have been men¬ 
tioned ; in addition the outer layers of the cells in stem, the spaces between 
the stem-leaves and the stem, the swollen auricular cells at the basal angles of 
the stem-leaves, the retort cells of the branches, and the spaces between the 
imbricated and concave leaves often serve for storing water. Some of these 
will be referred to with more detail later on. 
Many Sphagna have a compact or tufted habit which is more pro¬ 
nounced in plants of drier moorlands, e. g. S. compactum , S', cymbifoliurn 
var. congestum. 
Reasons for Protective Devices. 
It is difficult to reconcile the general watery habitat of Sphagnum with 
the presence of characters which, in other Mosses, act as xerophytic devices, 
and, as is evident from the comparison of different kinds of Sphagnum , also 
tend to act as protective devices with them. This tendency, however, is 
not always manifest; inconsistencies and anomalies are apparent if we 
attempt to make a comparative correlation of the structure of the plants 
and the wetness of their habitats. A species which is often found in a moist 
place, where it may be exposed to temporary dryness, is also found in a bog 
pool where wetness is permanent or almost so ; a species showing many of 
the foregoing devices may occur in an almost constant pool, whilst another 
with fewer or less prominent devices is in a temporary water-basin, so that 
one is almost tempted to conclude that there is no correlation, and that the 
species of Sphagnum have wide ranges of habitat. Careful field-work, 
however, shows that there is some selective power, some species favouring 
very wet, and others moist habitats; but still there must be other factors 
influencing the distribution besides the degree of wetness, and the most 
important of these is the acidity of the solution. 
It seems highly probable from recent work that Sphagna produce 
acids owing to the adsorption of the base by some substance of a colloidal 
nature present in the cell-walls, the amount of acid liberated being 
