The ventral portion of the archegonium is very early ruptured _at its base_ by the growing sporogonium, upon which it remains, and it is thus raised into the air, forming a “hood,” the calyptra (Figs. 192; 200 _B_). In the Sphagnaceæ the hood is not present; in this order, as in the Liverworts, the archegonium remains at the base of the sporogonium. The sporangium opens by circumsessile dehiscence, the upper portion (_operculum_) being separated along a specially constructed ring of cells, and falls off like a “lid” (Fig. 200). Only in a few forms (families 2 and 3) does any variation of this take place. Elaters are never found, but (with the exception of _Archidium_) there is always present in the sporangium a central mass of cells, the _columella_, which take no part in the formation of the spores. The columella, in some, does not reach quite to the operculum and in these cases the spore-sac is bell-shaped and covers the columella (_Andreæa_, Fig. 190; _Sphagnum_, Fig. 199 _D_); but in the majority of Mosses the columella extends to the lid, so that the space containing the spores becomes a hollow cylinder. The _sporangium_ is generally raised on a long stalk; in the great majority this stalk is formed from the lower half of the oospore and belongs to the asexual generation--it is then known as the _seta_. In _Andreæa_ and _Sphagnum_ the seta is very short, and the sporangia are raised upon a long stalk (_pseudopodium_) developed from the summit of the sexual generation (Figs. 190, 192). In the latter figure an archegonium (_a_) is seen attached to the pseudopodium, having been carried up with this during the course of its development. The summit of the pseudopodium is enlarged to embrace the foot of the sporogonium (Figs. 192, 199 _D_). A. The sporangium is supported on a pseudopodium; the columella does not extend to the operculum. [Illustration: FIG. 199.--_Sphagnum acutifolium._--_A_ The upper portion of a plant: _a_ branches with antheridia; _ch_ branches with terminal archegonia and perichætia; _b_ the upper stemleaves. _B_ A male branch whose leaves are partly taken off in order to show the antheridia. _C_ Group of three archegonia: the central one (_a_) is formed from the apical cell. _D_ Sporogonium in longitudinal section: the broad foot (_sg’_) is sunk in the vaginula, _v_; _c_ calyptra; _ar_ neck of the archegonium; _ps_ pseudopodium. _E_ ripe sporangium with operculum, and the remains of the archegonium situated on the pseudopodium which is still surrounded by the perichætium; to the left is a barren branch. _F_ Portion of a foliage-leaf seen from above: _l_ perforations; _b_ chlorophyll-containing cells; _s_ spiral thickenings.] Family 1. =Sphagneæ (Bog-Mosses).=