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SPORANGIUM and SPORANGE

(from spora, seed, and aggeion, a vessel; the latter word is sounded as if spelt angeion). Terms used to denote the small vessels or cases in which the spores of Ferns are produced, on the backs of the fronds, in the familiar, little, dark masses called sori. The Sporangia may be exposed on the surface of the fronds from their origin (e.g., in Polypodies), but are usually protected, more or less completely, under a membrane or indusium. Each Sporangium is supported on a rather slender stalk in most Ferns; but in a few they are attached without a stalk. In the Tribe Marattieae they may be partly joined to one another, and among Ophioglosseae they are sunk in the substance of the modified fertile frond. In most Ferns, they are formed of a single layer of cells, which are thin-walled, with the exception of a single row of thicker-walled cells, known as the annulus, which runs over the top of the Sporangium, or around it horizontally, or forms a cap on the top. Whatever its position, the annulus, by its resistance to pressure, causes the ripe Sporangium to split in a definite place. The spores in the other Orders of Vascular Cryptogams also are contained in Sporangia; and the same term is employed for corresponding structures among the Cellular Cryptogams; but for the peculiarities of their structure in these groups the reader must refer to works on Systematic Botany, as these peculiarities are of no special interest in gardening.