SPORANGIUM (Neo-lat., from Gk. eir6pos, sporos, seed + ey-yeZov, angcion, vessel). The plant organ within which asexual spores are pro duced. Among alga' and fungi the sporangium is usually a single cell (mother-cell), which pro duces few to numerous spores. Among bryophytes there are no distinct sporangia, the spores being produced by a more or less complex ea p.itile which is the essential feature of the characteristic leaf less sporophyte (sporogonium) of the group. Among pteridophytes the sporangia are very prominent, and their origin is the basis of a fundamental distinction in the group. They are complex, many-celled organs, usually borne on leaves, and if they are developed from a single superficial (epidermal) cell the plant is /epoto sporo»piuto, if they involve several epidermal and deeper cells the plant is euspormigiate. The an cient ferns, represented by a few tropical farms to-day, were eusporangiate; while the great host of modern ferns, including the water ferns, are leptosporangiate. It is natural. therefore, to regard the eusporangiate condition as primitive, and the leptosporangiate as derived. The two other divisions of pteridophytes (horsetails and club mosses) and all the spermatophytes (seed plants) are eusporangiate.
The structure of the ordinary fern sporangium indicates its highly specialized character. It consists of a long slender stalk that hears a spore-containing capsule. The walls of the capsule are thin, except for a single row of thick-walled cells (annulus) which girdles the capsule like a meridian. from the stalk nearlyaround to the stalk again. At maturity the annulus acts like a bent spring, and when the thin wall of the cap sule begins to yield straightens violently, hurling out a spray of spores. In the eusporan giate plants such a contrivance is absent.
In most ordinary ferns the sporangia are de veloped in very great numbers upon the under surface of foliage leaves, usually occurring in definite groups (sori), popularly called 'fruit dots,' which are generally protected by a flap-like outgrowth (indusium, q.v.) from the epidermis. In some ferns (e.g. 'sensitive fern'), however, foli age-work and spore-production are separated, and distinct foliage leaves and sporophylls ('spore-leaves') occur. This distinction persists
in the other groups of pteridophytes (horsetails and club mosses) and in the spermatophytes. the sporophyll being a constant organ in them. In the horsetails ( Equisetales, q.v.) and most of the club mosses (Lycopodiales. q.v.) the small sporo phylis are organized into a cone-like cluster (strobilus), which also appears as the so-called cones of pines, and as the equivalent in general of the flowers of angiosperms. In plants which exhibit heterospory (q.v.) the sporangia are dif fm.entiated, some producing megaspores (mega spo•augia) and others unerospores ( mierospo rangia). This differentiation begins among the pteridophytes, most notably in the club mosses ( Selaginella, q.v.), and is found in all seed plants. Since the two forms of sporangia in seed-plants, long called pollen sac5 and ovules, are really mierosporangia and megasporangia re spectively, stamens and carpels are properly sporophylls, and not sex organs, as commonly supposed.
The structure of a complex sporangium, such as occurs among all the vascular plants (fern plants and seed-plants), is constant in character though somewhat diverse in details. In the very early stages of a sporangium. when it consists of a mass of similar cells, there is no distinction of regions. Very early, however, a single cell or group of cells (archesporium) assumes the office of spore production, dividing more or less and producing in some eases a considerable mass of tissue. In any event, the cells of the last division are called the spore mother-cells, because within each one of them four spores are formed, the group of four being known as a tetrad. In a completely formed sporangium, just outside of the mass of mother-cells, there is a more o• less dis tinct nourishing layer called the tapetum, out side of which is the sporangium wall, consisting usually of two to five layers of cells, and various ly modified for protection, discharge. etc. This general account applies to all sporangia of the higher plants, excepting the mega sporangia or ovules of seed-plants, where the ordinary sporan gium structure is more or less modified, and the formation of the tetrad is obscured.