Lycopodiophyta (Microphyllophyta). The Selaginellaceae are terrestrial or epiphytic heterosporous, protostelic vascular plants comprising a single genus and about 700 species. The free-living sporophytic plant body is often somewhat dichotomously branching, consisting of a rhizome with adventitious roots, and an aerial portion with numerous, closely spaced, spirally disposed or 4-ranked and often dimorphic microphyllous leaves, each with a single vascular trace and each with a minute basal flap of tissue called a ligule. The fertile axis bears two kinds of sporangia. Megasporangia produce four large, megaspores that give rise to female gametophytes, and except for a small pad of tissue that bears archegonia, the entire gametophyte remains confined within the megaspore wall. Microsporangia produce hundreds of tiny microspores that give rise to male gametophytes, each bearing an antheridium contained within the wall of the microspore. Bursting of the microspore wall releases biflagellated sperm that must travel through a film of water in order to reach the egg of an archegonium of a nearby megagametophyte and initiate the zygote that may develop into a new sporophyte individual.
Each "thumbnail" image below is linked to a larger photograph.
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Selaginella sp., Akaka Falls, Hawaii. | ||
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Selaginella flabellata, small club moss. This is a species with dimorphic microphylls of two sizes on the shoots bearing the strobili. Strobili may be readily seen along the upper portion of the shoots extending to the left and right in the photo. Very close inspection of the strobili will reveal a few slightly 4-lobed megasporangia with more distended sporophylls among the more prevalent unlobed microsporangia. | ||
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Selaginella deflexa, small club moss. A 3-lobed megasporangium is clearly visible inside the bend in the fertile axis. The smooth, unlobed sporangia are microsporangia. The megasporangia contain 4 large spores in a tetrahedral configuration while the microsporangia contain hundreds of tiny microspores. |
