538 MR. W. H. BURRELL ON LEUCOBRYUM GLAUCUM, SCHP. 
it had been there long enough to be washed free from earthy 
matter and for its base to conform to the inequalities of the 
underlying moss, but obviously it had not originated there, 
for the Mnium was only slightly discoloured. 
There are, I believe, three contributory causes. 
(1) Special cell structure enabling the plant to retain 
moisture. 
(2) The formation of vegetative buds from rhizoids on the 
leaves. 
(3) Repeated accidental disturbance. 
The latter is the predisposing cause, but the radiate growth 
which follows is dependent upon the two first named details 
in the plant economy. 
With comparatively few exceptions the leaves of mosses 
are formed of a thin lamina consisting of a single layer of cells, 
with or without a nerve, the large surface exposed accounting 
for the rapidity with which these plants shrivel in dry 
atmosphere and reabsorb moisture from damp air. In 
Leucobryum there is a great development of the nerve at the 
expense of lamina, which is reduced to a few rows of cells at 
the leaf margins. A transverse section shows a central layer 
of small chlorophyllose cells embedded in two to four layers 
of large empty hyaline cells whose communicating walls are 
porous ; when dry these hyaline cells are filled with air and 
obscure the colour of the green central layer, hence the name 
Lenco bryum, white moss ; after rain the air is displaced by 
water, which can be squeezed out as from a sponge, and the 
green colour of the chlorophyllose cells becomes clearly 
visible. The leaves retain moisture for a long time, and 
maintain their vitality under conditions which would arrest 
growth in plants with one layered leaves. Braithwaite refers 
to the tendency of the plant to mature its capsules even in 
the drying press and recommends dipping in boiling water 
to prevent dehiscence. 
Leucobryum rarely fruits ; reproduction is effected in 
general by buds rather than by spore formation ; reddish 
brown tomentum forms on the surface of the leaves, and 
from this rise buds which develop into new plants. This 
