Watson.—Sphagna : Habitats , Adaptations , and Associates. 547 
Further Characters important in Relation 
to Water-supply. 
Besides the characters previously mentioned there are others which are 
worth considering in relation to the water-supply : 
1 . The relative positions , shapes , of the chloraphyllous and 
hyaline cells , particularly of the branch leaves. (#) In .S’, sub secundum and 
its allies the two kinds of cells are almost rectangular in shape; the chloro- 
phyllous cell is medianly placed between the contiguous hyaline cells, and is 
more or less exposed on both the inner and outer surfaces of the leaf 
(Fig. 3, K ; Fig. 5, V), so that the only protection afforded to it by the hyaline 
cells is due to the fact that the thickness (antero-posterior diameter) of the 
hyaline cell is greater than that of the barrel-shaped chlorophyllous one 
and so to some extent overlaps it, the surface of the latter being less than 
the average area of a cross-section. The species of this section are founded 
on differences between the characters of the upper branch leaves ; the lower 
branches are usually immersed in water, have less distinctive characters, 
their chlorophyllous cells are wide, more or less rectangular, and little over¬ 
lapped by the hyaline cells. In 5. obesum , the most hydrophytic member, 
there is often little difference in size, shape, and position of the two kinds of 
cells (Fig. 5 , V). 
The median position of the chlorophyllous cell is also found in 
S. papillosum (Fig. 3 , p) and* S. medium (Fig. 4, K, L, M), plants of higher 
acidity and drier situations, but the chlorophyllous cell is more enclosed by 
the hyaline, and other methods (e. g. imbricated and cucullate leaves) for 
retaining or obtaining water are more prominent in these plants. 
(b) In S. cuspidatum (Fig. 4, E), .S', squarrosum , and their allies the 
chlorophyllous cell is triangular to sub-rectangular with its broader end 
exposed on the outer surface of the leaf. The outer walls of the hyaline 
cells are slightly convex but afford little shade to the surface of the chloro¬ 
phyllous cell; the inner walls are. often more convex and overlap the 
chlorophyllous cells, but little protection against normal desiccation is 
afforded by this structure, so that practically the only advantage accruing 
to the living cell by the proximity of the hyaline one is that the latter acts 
as a reservoir. Under strong desiccating influences evaporation may occur 
on both the inner and outer surfaces and then the enclosure of the chloro¬ 
phyllous cell on the inner surface may be beneficial. 
5 . cuspidatum (Fig. 4, E), with large sub-rectangular chlorophyllous 
cells free on both surfaces of the leaf, has the most aquatic environment; 
5 . pulchrum (Fig. 4, g), with the cells partially enclosed, has the driest 
habitat; whilst 5. recurvum (Fig. 5 , C) and 5. squarrosum are intermediate 
both in structure and habitat. .S. compactum has its chlorophyllous cells 
