J52 
W. Watson. 
The cells of mosses are capable of withstanding long periods 
of drought or cold without being killed. This power is much more 
evident in bryophytes (and still more in lichens) than in the 
higher plants and may be considered to be a direct effect of life in 
dry or cold air rather than a xerophytic character. The same 
power is shown to a varying extent in higher organisms (17), and 
is correlated with the degree of interdependence amongst the cells, 
though modifications may be induced, either naturally or artificially, 
by altering the environment of the plant. The setae of a moss 
grown in a damper atmosphere than its usual habitat are much 
sooner killed by drying than those produced under normal conditions. 
Irmscher has carried out many interesting and instructive experi¬ 
ments on drought and cold resistance in mosses ; for details the 
reader may be referred to his paper (10). 
A bryophyte which is capable of drying up without dying is 
therefore not necessarily a xerophyte ; it must be able so to with¬ 
stand long periods of dryness that sufficient cells are alive to enable 
the plant to resume its growth quickly when water becomes available, 
and to do this it must possess some of the xerophytic characters 
mentioned in the following pages. 
Many small earth-mosses are annual species, their activity 
being only displayed in the moister half of the year, passing over 
during the drier portion as spores or underground protonema. 
The chief protective arrangements against drought are shown 
in the gametophyte, which is usually perennial; the sporophyte has 
a much shorter life, especially in the Hepaticee, its function being 
to produce and scatter the spores. 
II. Protective Arrangements in the Sporogonium. 
During its development the sporogonium requires some protection, 
but when it is mature in the moss few protective devices are necessary, 
as it is built up of hard and firm tissue, the outer skin (exothecium) 
of the urn has its outer walls cuticularised, there is an intercellular 
space around the spore-sac, and the seta also has an epidermis of 
cuticularised cells. During the early days of the moss sporogoninm 
its young and slender tissues are protected by the perichaetial leaves 
of the gametophyte, and covered by the enlarged archegonial wall. 
When this wall bursts, the upper portion forms a bonnet (calyptra) 
which protects the unripe urn from dryness till its outer walls 
become cuticularised. In some mosses the perichaetial leaves are 
much longer than the ordinary vegetative ones, e.g., Seligeria pusilla, 
S. acutifolia, Archidium alternifolium, species of Pleuridium (Fig. 1, 
