The =Sporangia=. The asexual generation of the _Mosses_ is the sporogonium, in which the spores arise in tetrads from the mother-cells. The sporangia in the _Filicinæ_ take their origin either from a single cell (Leptosporangiatæ) or, what probably may be regarded as an older stand-point, from a group of cells (Eusporangiatæ). In both cases there may be distinguished in a mature sporangium three tissues, which have different significance (Fig. 204): (1) an external layer, the _sporangium-wall_, most frequently composed of one layer of cells made up of cells of dissimilar structure, so that on desiccation the wall is ruptured and the sporangium opens in a definite manner; (2) an internal group of cells, consisting of the _spore-mother-cells_, developed from an archesporium, and which by division into four gives rise to the _spores_; (3) a layer of cells lying between the two already mentioned, which is dissolved before maturity. The intermediate cellular layer, which directly surrounds the spore-forming cells, is in form and contents more worthy of note than the others, and is termed the _tapetum_. The construction of the sporangium in the _Equisetinæ_ and _Lycopodinæ_ is in the main the same. [Illustration: FIG. 247.--Development of an anther. _A_ Transverse section of a young anther of _Doronicum macrophyllum_. The formation of the 4 pollen-sacs commences by divisions of the hypodermal cells (at _m_, for instance). These cells divide by periclinal walls into external cells which only take part in forming the anther-wall; and internal cells, which correspond to the Archesporium, and from which the spores are derived. These spore-forming cells are drawn with thicker walls in _B-E_. The commencement of the vascular bundle is seen in the centre. _B_ An older stage; the pollen-sacs already project considerably. It is the cells in the hypodermal layer which are active and in which tangential divisions particularly occur; _fv_ vascular bundle. _C_ A corresponding longitudinal section. _D_ Transverse section through an older anther, the thickness of the wall outside the mother-cells of the pollen-grains is already increased, and it becomes still thicker by the division of the hypodermal cells: its most external layer of cells but one, becomes transformed into the “fibrous cells.” _E_ Transverse section of a still older pollen-sac of _Menyanthes_; _sm_ are the mother-cells of the pollen-grains surrounded by the tapetum (_t_), external to the tapetum is the anther-wall, which is still far from being fully developed. The sub-epidermal layer becomes “fibrous,” and the cells lying inside it become dissolved, together with the tapetum.]