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The perianth is an organ peculiar to the scale-mosses, although not 
found in all of them. In the majority of our species it develops whether 
fertilization takes place or not; in a few species, however, its development 
depends upon fertilization. In no case does the development of the perianth 
precede that of the archegonia. The perianth consists of a hollow tube, which 
is attached at the base and open at the tip, sometimes with a wide mouth. 
Under normal circumstances it surrounds the young sporophyte and assists 
the calyptra in protecting it {Fig. 24). Most writers look upon the perianth as 
a structure formed by the coalescence of leaf-like organs, and it differs in 
appearance according to the number and character of the leaves which enter 
into its formation. In the simplest case it is formed by the union of two leaves 
and one underleaf, which remain flat. This gives rise to a perianth in the 
form of a triangular prism, three angles or keels being formed by the united 
edges of the leaves. If we suppose that such a perianth is pressed back 
against the substratum, two of the keels will be lateral and the third will be 
dorsal. This type, which is known as “ epigonianthous,” is beautifully 
shown in the genus Lophocolea, and may be represented in cross-section by 
such a diagram as Fig. 25. In case no underleaf takes part in the forma- 
tion of the perianth, we observe a structure which is compressed at right 
angles to the substratum, a condition found throughout the large genus Plagi- 
ochila {Fig. 26). A very different type of perianth arises when the 
leaves which enter into its formation are complicate instead of being flat. In 
this case the keels of the leaves usually form keels on the perianth, and when 
a triangular perianth is formed, the third keel instead of being dorsal will be 
ventral. This condition, called “ hypogonianthous,” is found with certain 
modifications in a large number of genera, of which Frullania , Lejeunea , 
p or el la, Cephalozia , Bazzania and Lepidozia may be especially mentioned 
(Fig. 27). When no underleaf takes part in this type of perianth, we 
again observe a compressed structure, but this time the flattening is parallel 
to the substratum instead of at right angles to it {Fig. 28). This condition 
is found in the genera Radula and Scapania. In many species the structure 
of the perianth is obscured, either by the obliteration of keels or by the inter- 
polation of additional keels, and under these circumstances the interpretation 
becomes much more difficult. 
The perigynium, unlike the perianth, is formed directly from the arche- 
gonial branch. It occurs in comparatively few genera, and its development 
is always dependent upon fertilization. While the sporophyte is growing, 
the archegonial branch which bears it begins to grow also in the form of a 
hollow tube. This encloses the young sporophyte and carries up on the out- 
side the bracts and bracteoles. In some cases, as in Nardia and Marsu- 
pella, the perigynium bears a perianth at its mouth (Fig. 2 q). In other 
cases, as in Gymnomitrium, there is no perianth formed. In the examples 
so far considered the perigynium has grown in an upward direction only : in 
other cases, however, it grows downward as well , and sometimes its growth 
is entirely downward. The first of these conditions may be seen especially 
well in Nardia haematosticta, the second in Geocalyx and Kantia , the peri- 
gynium in these two genera being in the form of a pendent sac (Fig. jo), 
