The President's Address. By Dr. R. Braithwaite. 139 
in the Leucobryaceae the cells are dimorphous and in two strata. On 
the stems of many pleurocarpous species we find, intermixed with the 
leaves, smaller leafy organs irregularly lobed or cut up into cellular 
threads ; these are named paraphyllia. The leaves may be entire or 
serrated or ciliate, and occasionally are surrounded by a thickened 
border or limb of very narrow cells. The surface is smooth or papil- 
lose, and in the aloid Tortulae the upper side is covered with jointed 
threads, while in the Polytricha the upper surface of the nerve is 
beset with vertical laminae, thus largely extending the assimilating 
area. The position of the leaves when dry is appressed, twisted 
spirally or cirrate, but when moist they become erecto-patent, 
spreading or recurved. The cells composing the leaf-lamina are of 
the highest importance in distinguishing the genera and species of 
mosses, and their form and arrangement should be carefully noted by 
all who study this extensive group of plants. 
Two principal forms may be distinguished : (1) parenchymatous, 
quadrate or hexagonal, sometimes becoming roundish, but their ends 
are always obtuse, and they are mostly arranged in rows ; (2) pros- 
enchymatous, are always more elongated, rhombic, or linear, with 
their ends triangular ; sometimes they are vermicular or twisted, and 
they often compose the areolation of the upper part of a leaf, while 
that at the base is parenchymatous. The basal angular, or alar cells 
are of importance in the Hypnaceae, and are parenchymatous with 
wide lumen, and form one or two strata. Cell-walls often become 
thickened by internal deposit, by which the cell-lumen is diminished 
in size and dot-like as in Andredea, Grimmia, Barhula , (fee., and 
occasionally nodules and papillae form on the internal walls, as in 
Grimmia sect. Trichostomum, and several Sphagna. Pores may also 
be noticed in the walls of leaf-cells by which they communicate, well 
seen in Dicranum scoparium and in the transverse partitions of 
Leucobryum. The leaf-cells contain a granuliferous plasma, the outer 
layer of which (the enveloping layer, until recently called the primor- 
dial utricle) is firmer and applied to the inner surface of the cell-wall, 
and besides the chlorophyll- granules, oil-globules are often present. 
The midrib gives the leaf firmness, and on section is seen to consist 
of several cell-layers ; generally a row of wide, thin- walled cells occu- 
pies the centre, attended by groups of similar smaller cells and 
supported by stereid cells which are given off from those in the stem, 
just as the fibro-vascular bundles in the leaves of flowering plants. 
The midrib projects more or less at the back, where it sometimes 
bears 2-5 wing-like lamellae, sometimes several rows of serratures. 
W ater passes up the central pith of the stem and along the midrib to 
the leaves, where it is transpired, but to a greater extent it takes place 
the reverse way, being taken up directly by the leaves and the rbizoids 
of the stem. The inflorescence varies in position and its constituent 
elements ; it is terminal when it ends the main axis, and lateral when 
developed on an abbreviated lateral shoot ; dioicous when male and 
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