162 
W. Watson. 
( d) Protective Layer of Dead Cells , with the chlorophyllous cells 
between them. In Leucobryum, a moss growing on heaths and turfy 
ground the chlorophyllous cells are covered except at the margin 
with two or more layers of rectangular hyaline cells whose internal 
walls are perforated with large pores, the internal chlorophyllous 
layer consisting of narrow and small cells, so that the plant has a 
whitish appearance. The presence of the pores renders absorption 
so easy that the moss may be said to suck up water like a sponge, 
and during dry weather, there is no doubt that the empty external 
cells act, not only as water-reservoirs, but also as screens against 
the warmth from the sun and the drying action of the wind. 
Goebel (7) describes a similar apparatus in the foreign moss 
Syrrhopodon. In the Sphagna , which are usually regarded as hydro- 
phytic plants, and grow in bogs, we also get perforated hyaline cells, 
and it is significant that the plants growing in less “acidic” waters 
have greener leaves and fewer pores (e,g., Sphagnum subsecundum 
v. viride, S. riparium , 5. cuspidatum). 
(e) Infolding of the Leaf-margins. In Polytrichmn spp. the wings 
of the leaves consist of hyaline cells and bend over the assimilating 
portion of the leaf (midrib) when the air is dry, so that only a small 
chink is left for the gas-exchange. This feature is specially marked 
in the species commonly found in dry or exposed places, e.g., P. 
piliferum, P. sexangulare , P. juniperinum. 
(/) Bordered Leaves. Many species of mosses, have the margin 
of the leaf occupied by several rows of long empty cells, which not 
only serve as water-reservoirs, but also decrease the evaporating 
surface. These cells are the first to become desiccated, and then 
form a protective layer to the rest of the leaf, becoming more 
conspicuous during drying owing to the curling, crisping and curving 
of the leaf, thus acting as a screen to the underlying chlorophyllous 
cells. They are usually found in mesophytic plants with large cells, 
and compensate for the larger amount of transpiration that may 
take place owing to the size of the cell-cavities, e.g., Funaria erice- 
torum, Tortula subulata, Webera Tozeri, and some species of Daltonia, 
Bryum, Mnium, Hookeria, and Sphagnum. 
Many mosses, which do not usually possess this character, 
often show narrower and less chlorophyllous cells at the leaf-margin, 
when they are growing in drier situations, e.g., Physcomitrium pyri - 
forme , Webera nutans, Funaria fascicularis. 
In the liverwort Aplozia crenulata the outer cells of the leaf- 
margin are larger and less chlorophyllous, and as forms which grow 
