548 Watson.—Sphagna : Habitats , Adaptations , and Associates. 
almost median but nearer to the .outer surface (Fig. 4, 1). Its densely 
tufted and compact habit, its closely imbricated and slightly cucullate leaves, 
enable it to grow in drier situations than any of the above, though its var. 
sqnarrosum with less crowded branches and more or less squarrose leaves 
has a wetter habitat than the type. 
( c ) In cymbifolium (Fig. 5, J, K, L), 5 . imbricatum , 5 . fimbriatum , 
and the Acutifolia group (Fig. 2, j), the chlorophyllous cells are triangular 
in shape, the base of the triangle being on the inner side of the leaf, whilst 
the outer walls of the hyaline cells are so convex that they entirely or almost 
entirely enclose those containing chlorophyll. This arrangement is an 
excellent device for lowering the illumination and for preventing excessive 
transpiration, and the Sphagna which possess it grow in situations exposed 
to light and with an inconstant water-supply, or in water which is strongly 
acid at times. These plants are seldom shaded ; if they are, compensating 
characters such as looser or squarrose leaves (e. g. S', cymbifolium var. 
squarrosidum) are shown. As the Sphagna with median cells are more 
primitive in this respect, the displacement to the outer or inner surfaces 
may be regarded as due to light as well as to water requirements. 
The relative sizes of the two kinds of cells must also be considered. 
The water-supply will be more profuse when the ratio 
Volume of hyaline cell 
Volume of chlorophyllous cell 
becomes greater if we consider cells with similar positions and shapes, and 
when there is any overlapping of the hyaline cell over the chlorophyllous 
one, the shading will be greater the narrower the latter is ; in 5 . obesum 
and 5 . medium the chlorophyllous cell is median, but is wider in the former, 
a plant of wetter habitat (cf. Fig. 5, T, V ; and Fig. 4, K, L, m). 
The stem-leaves also have variations similar to those of the branch- 
leaves, though of much less extent (Fig. 2, c, D, E. F). As they are usually 
concealed by the branch-leaves they have less chlorophyll; there is often 
little differentiation between the two kinds of cells, pores and fibres are fewer 
or absent, and in most cases their chief function is to assist the pendent 
branches in supplying water to the upper parts of the plant. The perichae- 
tial and perigonial leaves are often more primitive in their structure and the 
differentiation into two kinds of cells is often incomplete (Fig. 3, C, D, F, g). 
2. Position , size, number , and bordering of pores in the hyaline cells , 
especially in the leaves of the upper divergent branches. 
The hyaline cell usually possesses fibres to maintain its shape, and its 
membrane is usually perforated to form pores. The usual form of pore is 
surrounded by a distinct thickened ring of cell-membrane which keeps it 
open, but non-bordered perforations also occur, especially on the lower 
portions or the inner surfaces of the leaf. The chief function of these pores 
