Watson. — Sphagna: Habitats, Adaptations , Associates. 549 
is the rapid intake of water, but they are also of use in promoting a readier 
diffusion of acids, and in some cases (especially in the Subsecunda section) 
a considerable amount of evaporation may occur through the pores on the 
outer surface. Their disposition varies according to the species: 
(a) Pores chiefly on the outer surface of the branch leaf. 
1. Pores large and few in each cell :—Cymbifolia section (Fig. 5, j, K ; 
Fig. 4, K, L). In S. squarrosum and S. teres the pores are large and few, 
but the porosity of the inner is similar to that of the outer surface. 
2 . Pores moderate and few in each cell:—Acutifolia section (Fig. 2, H, I), 
S. compactum. 
3. Pores small and many in each cell:— S. subsecundum, .S’. inundatum 
(Fig. 5, G, h), S. rufescens (Fig. 5, x, y), .S'. auriculatum. In 5 . rufescens 
the pores are often numerous on the inner surface as well. 
(b) Pores small and many, chiefly on the inner surface of the branch 
leaf:—.S'. crassicladum, S. molluscum. In the former the outer surface 
shows thickenings (pseudopores), indicating that it has been evolved from an 
ancestor possessing pores on the outer surface. 
(e) Pores absent or few on both surfaces, if present small and usually 
on the inner surface :— S.cuspidatum , A. rip avium, S. obesum, S. recurvum, 
S. amblyphyllmn, all of which have an abundant water-supply (Fig. 4, c, D ; 
Fig. 5 . T). 
The potes on the outer surface facilitate the rapid intake of surface 
water (as after rain), are usually bordered and so kept open. Evaporation 
of water may occur through them, and will be greater when the pores are 
numerous and small than when few pores have an equal area. When the 
leaves are more or less imbricated the pores on the upper exposed portions of 
the outer surface will be more efficient for obtaining external water, and these 
places show the most characteristic pore formation. The pores on the inner 
surface are favourably situated for interchange of substances and for obtain¬ 
ing water from the dilute solution passing along the axis of the plant; they 
are present though few and scattered in S. recurvum and S. squarrosum 
(with large pores), which obtain their main water-supply from below, but are 
best shown in the upper branches of S. crassicladum, a plant which is often 
immersed or almost immersed in streamlets or pools of fresh water. 
The pores on the inner surface of the leaf or on the lower part of the 
outer surface overlapped by an imbricated lower leaf are usually weakly- or 
non-bordered, are often larger, and allow ready interchange of substances 
with the axial water-supply. The absence or paucity of pore formation is 
correlative with an abundant water-supply. 
3. Large superficial hyaline cells of the stem. The presence of these 
maybe considered a generic character, as in all species there are one or 
more cortical layers of large, thin-walled, hyaline cells surrounding a 
stereom ring of smaller and thicker walled cells and a central core of thin- 
