550 Watson. — Sphagna: Habitats, Adaptations, and Associates. 
walled and fairly large cells. The only cells containing chlorophyll are 
immediately underneath the superficial hyaline cells. The following types 
occur : 
(a) The hyaline cells are in two or three layers but are not very distinct 
from the more internal cells, being only slightly larger and thinner 
walled :—.S', recurvum (Fig. 5, B), 5 . amblyphyllum, S.obtusum. In 
vS. riparium , S. compaction, and .S', pulchrum they are often some¬ 
what distinct, and therefore these plants may be better placed in (c). 
(b) They form one layer of large cells (with few or no pores) clearly 
differentiated from the stereom cylinder:— 5 . subsecundum and its 
allies (Fig. 3, j). 
(c) They form two to five layers of large cells clearly differentiated from 
the stereom cylinder: —.S', acutifolium and its allies (Fig. 3, a), 
5 . squarrosum, S. teres, S. cuspidatum (Fig. 5, p), 5 . molluscum, 
S. cymbifolium (Fig. 5,. m) and its allies. Pores are few or absent 
(Fig. 3, B) except in the Cymbifolia group, which also have strength¬ 
ening fibres in the cells (Fig. 3, N). 
The plants of the first two types have an abundant water-supply, and 
have a low acidity, whilst most of the plants of the third type have a high 
acidity, and at times are exposed to drier conditions. The hyaline cells of 
the stem and their contained pores have similar functions to those of the 
leaf. The fibres similarly serve as strengthening bodies which cause the cell 
to retain its shape so that water maybe quickly absorbed. In the leaf the 
fibres are often weak or absent in parts where there is a constant water- 
supply. Other things being equal the plants with pores and fibres are 
more able to withstand exposure. 
4. Other hyaline cells for the storage of water are present in the retort 
cells and leaf auricles. 
The retort cells are large flask-shaped cells superficially placed on the 
outside of the hyaline cells of the branch ; their necks often curve away 
from the axis and are perforate at the ends. They vary according to the 
species, sometimes being absent, and reach their highest development in 
5 . molluscum (Fig. 5, E, cf. d). 
The basal angles of the stem-leaf are usually occupied by larger hyaline 
cells, and sometimes these are extended into little outgrowths known as 
auricles (Fig. 3, H, i). e. g. 5 . auriculatum , 5 . inundatum. 
Sporogonial Plant. 
As the sporogonium is overtopped by the perichaetial leaves till it is 
almost mature, little further protection is needed by it. The calyptra is 
small and quickly disappears when the false seta elongates. The sporogonia 
then soon begin to ‘ pop ’ and the spores are scattered. In some species the 
