dry weather the margins of the leaves roll inward, and the leaves fold closely against the stem, thus protecting the delicate assimilating tissue.
The antheridia and archegonia of polytrichum are borne in groups at the ends of the branches on different plants (many mosses bear both organs on the same branch). They are surrounded by involucres of characteristic leaves termed perichætia or perichætal leaves. Multicellular hairs known as paraphyses are scattered among the archegonia and antheridia. The involucres with the organs borne within them are called receptacles, or, less appropriately, "moss flowers." As in marchantia, the organs are very minute and must be highly magnified to be studied.
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Fig. 297.—Section through a Receptacle of Polytrichum commune, showing paraphyses and antheridia.
The antheridia are borne in broad cup-like receptacles on the
antheridial plants (Fig. 297). They are much like the antheridia
of marchantia, but they stand free
among the paraphyses and are not
sunk in cavities. At maturity they
burst and allow the sperm cells or
spermatozoids to escape. In polytrichum,
when the receptacles have
fulfilled their function, the stem continues
to grow from the center of
the cup (m, Fig. 295). The archegonia
are borne in other receptacles
on different plants. They are like
the archegonia of marchantia except that they stand erect on the
end of the branch.
The sporogonium which develops from the fertilized egg is shown in a, b, Fig. 295. It consists of a long, brown stalk bearing the spore-case at its summit. The base of the stalk is imbedded in the end of the moss stem by which it is nourished. The capsule is entirely inclosed by a hairy cap, the calyptra, b. The calyptra is really the remnant of the archegonium, which, for a time, increases in size to accommodate and protect the young growing capsule. It is finally torn loose and carried up on the spore-case. The mouth of the capsule is closed by a circular lid, the operculum, having a conical projection at the center.
The operculum soon drops, or it may be removed, displaying a fringe of sixty-four teeth guarding the mouth of the capsule. This ring of teeth is known as the peristome. In most mosses the teeth exhibit peculiar hygroscopic movements; i.e. when moist they bend outwards, and upon drying curve in toward the mouth of the capsule. This motion, it will be seen, serves to disperse the spores gradually over a long period of time.
Not the entire capsule is filled with spores. There are no elaters, but the center of the capsule is occupied by a columnar
