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the cell-lumen disappears. These are termed stereid cells, and are 
of use in affording firmness and support to the stem. The walls of 
the interior cells are often perforated by small pores, by means of 
which the cells communicate. The main axis or stem is almost 
always branched, the shoots or innovations being vegetative only 
or sexual, and their place of formation is important as it charac- 
terizes the two main divisions of mosses : Acrocarpi, when given off 
at the crown or youngest part of the stem; Pleurocarpi, when 
produced laterally, from the base or older part of the stem, and 
thus compared by Limpricht to the cyme and raceme of flowering 
plants. The main axis in the Acrocarpi usually ends in the inflor- 
escence, male or female, and an innovation is given off laterally below 
the crown, on which the same process is repeated, so that the growth 
of the axis is limited ; often, as in Bnjum , two or three innovations 
are produced under each inflorescence. In the Pleurocarpi the in- 
florescence is thrown off first near the base on a short lateral branch, 
and lateral shoots are progressively produced upward, so that the 
main axis continues its growth in length, and the lateral shoots 
repeat the plan of the main axis, and the stem becomes pinnate, bi- 
or tripinnate, as in Thyidium. Sometimes the axis is continued as a 
naked shoot or pseudopodium terminated by a cluster of gemmae, as 
in Gymnocybe palustris , Georgia pellucida , &c., and sometimes it 
throws out at the base stolons or runners which creep on or below the 
earth and then throw up erect leafy shoots as in Climacium and 
Bryum proliferum. In many species of Nechera the lateral branches 
are attenuated and flagelliforin at the ends. 
Stems are usually erect, but in pleurocarpous mosses often creep- 
ing, sometimes pendent as in Meteorium, sometimes floating as in 
Fontinalis ; they also generally grow aggregated in tufts or dense 
cushions, rarely scattered. 
We have next to consider the leaves of mosses — those cellular 
expansions which adorn the stem and are so infinitely varied in form 
and structure, and, according to the mode in which the papillae are 
given off from the three-sided apical cell, do we get the phyllotaxy or 
arrangement of the leaves on the stem. This is really spiral, and 
produced by a twisting of the terminal cell in process of growth. 
When the leaves are most distant we have a bifarious arrangement or 
1/2, that is, two leaves in one spiral turn; or trifarious (1/3), three 
leaves in each turn ; but if the leaves are more approximate the 
number of them in a spiral increases, and we have 2/5 or 3/8 — which 
are most frequent. 
The leaves are attached transversely to the stem, and are never 
lobed or divided ; in form they vary between orbicular and subulate, 
but an ovate or lanceolate outline may be looked upon as most fre- 
quent. The lamina is generally traversed by a midrib or nerve of 
greater or less extent, and sometimes by two, and its cells are uni- 
stratose, but occasionally we find two strata in the upper part ; 
