Page:The New International Encyclopædia 1st ed. v. 18.djvu/541

* SPORANGIUM. 465 SPORE. SPORANGIUM (Xeo-Lat.. from Gk. airbpot, sporos, seed + dvYeroi/, anycioii, vessel). The plant organ within wliioli asexual spores are pro- duced. Among alga' and fungi the sporangium 13 usually a single cell (niother-cell ), wliieh pro- duces few to numerous spores. Among bryophytes there are no distinct sporangia, the spores being produced by a more or less complex capsule which is the essential feature of the characteristic leaf- less sporophyte (sporogonium) of the group. Among pteridnphytes 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 jdant is Irpolo- S}ii)fuiiiti(itc, if they involve several epidermal and deeper cells the plant is eiisporanyiale. The an- cient ferns, represented by a few tropical forms today, 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 bears a epore-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 TYPES OF ePORANOIl. 1, Quillwort; 2, fern; 3, liverwort: 4, club moBs; 5, fungus. meridian, from the .stalk nearly around to the stalk again. At maturity the anmilus 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 sporophylla ('spore-leaves') occur. This distinction persists in the other groups of ]ili'rid(ipliytes (horsetails and club mosses) and in tlic spermatophytes, the sporophyll being a constant organ in them. In the horsetails (Equisetales, q.v.) and most of the club mosses (Lyco])odialcs. q.v.) the small sporo- phylls 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- ferentiated, some producing megaspores (mega- sporangia) and others microspores (mi<'rospo- rangia). This difi'erentiation Ix'gins among the ])teridophytes, most notably in the clul) mosses (Selaginella, q.v.). and is found in all seed- plants. Since the two forms of sporangia in seed-plants, long called pollen sac.^, and ovules, are really microsporangia and niegasporangia 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 jilants (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 oases 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 or 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 sjjorangia of the higher plants, excepting the niegasporangia or ovules of seed-plants, where the ordinary sporan- gium structure is more or less modified, and the formation of the tetrad is obscured. SPORE (from Gk. airipos, sporos. seed). In general, a single cell separated from the parent plant for reproduction. In the simplest plants spores are not produced, as in the blue-green algiE (Cyanophycese, q.v.). but in all other plants they are a prominent method of reproduction, and one or other of two great groups that differ in their mode of origin. Asexual spores are usually pro- duced by mere division of the cells in a sporan- gium. Sexual spores are produced by the fusion of two sexual cells (gametes), which arise in a simple or complex organ called a gametangium (q.v.). A.SEXUAL Spores. The diverse names given to asexual spores usually indicate some peculiar character of the spore or the group of plants which produce it. Among the green algiP (Chlo- rophycefe, q.v.) the characteristic spore is a zo- os])ore or swarm spore, a minute, pear-shaped naked mass of protoplasm, swimming freely by means of one or two cilia at one end (fig, 9). Among the brown algse (Ph.Tophyce.T, q.v.) the zo<")spores are bean-shaped, with two cilia on the concave side. Among the red alg.-e (Rhodophy-