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Selaginella Strobilus

The genus Selaginella is classified in the division Lycophyta, which includes the numerous plants commonly known as clubmosses and spike mosses. Similar to their close relatives, Selaginella plants develop strobili, spike-like reproductive organs that form on fertile branches.

Selaginella Strobilus

Selaginella are unique, however, from other lycophytes in that their strobili contain two different types of spores. The spore cases near the bottom of a vertical strobilus have a lumpy appearance and are generally lighter in color than the sporangia near the apex of the structure. Within each of the basal spore cases are only four spores, but these spores, termed megaspores, are quite large. They develop from the meiosis of a single sporocyte. In contrast, the apical spore cases exhibit a smooth oval form and each contain numerous very small spores referred to as microspores that are produced by multiple sporocytes.

When the sporangia of a Selaginella strobilus reach maturity, they split open and release the spores they contain into the air. The megaspores give rise to female gametophytes known as megagametophytes and the microspores develop into male microgametophytes. The microgametophytes produce sperm equipped with flagella that enable them to swim to the eggs generated by a megagametophyte through water, either in the form of dew or rain, in order to achieve fertilization. Resulting from this process is the dominant form of Selaginella and other lycophytes, the sporophyte.

Contributing Authors

Nathan S. Claxton, Shannon H. Neaves, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.