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Starting with the sporophyte (1), some branches bear at their tips
aggregations of sporangium-bearing leaves, called sporophylls. They form
loose, conelike structures called strobili (2). Each strobilus usually
contains two kinds of sporophylls, microsporophylls bearing
microsporangia and megasporophylls bearing megasporangia. The outer wall
of the sporangia is a sterile jacket of cells that surrounds and
protects the enclosed sporogenous tissue. In the microsporangia, the
diploid sporocyte cells undergo meiosis to form numerous meiospores
which in turn form haploid microspores. In the megasporangium, the
entire contents of the megasporangium forms one large megasporocyte
that, by meiosis, produces four haploid megaspores.
Upon rupture of mature microsporangia, the small, lightweight microspores (3) are readily dispersed by air currents but the larger, heavier megaspores often remain in place in the strobili. Germination of a microspore produces a male gametophyte (microgametophyte) that develops totally within the confines of the microspore wall (endosporic). At maturity, the entire male gametophyte consists of a one-cell-layer-thick antheridium (4) and fertile tissue that forms biflagellated sperm. Sperm are released by rupture of the microspore wall (6). Megaspores (3) germinate to begin development of a female gametophyte (megagametophyte) (5) within the confines of the megaspore wall (endosporic). Absorptive hairlike structures, rhizoids which absorb water and dissolved minerals, develop on the female gametophyte. Archegonia also develop on the exposed upper surface of the female gametophyte. Each archegonium contains a single egg, and at maturity, the neck and ventral canal cells disintegrate to form an open passageway to the egg (7). Sperm, attracted by substances released from the matured archegonlum, swim through a film of water and enter the open passageway. One sperm eventually fertilizes the egg to form a zygote (8). Development of the embryonic sporophyte occurs within the archegonium in the female gametophyte. Durlng its early development as an embryo, the sporophyte is parasitic upon the female gametophyte. Breakdown of the parent sporophytic strobili releases the germinated megaspores (female gametophyte), and the young sporophytes soon develop a root and shoot system to become independent plants (9). |
