An order of the *Lycopsida
containing one extant genus, Selaginella, the 700 or so species of which are mainly tropical in distribution. In gross morphology Selaginella resembles Lycopodium (see Lycopodiales
) except that it has ligulate leaves and the sporophylls are always grouped in strobili. The leaves are usually inserted spirally but in some species they are arranged in four ranks, the upper two consisting of small leaves adpressed to the stem while the lower two ranks contain expanded leaves. The roots are borne at the end of leafless branches (see rhizophore
). Internally, selaginellas are distinguished by their trabeculate endodermis, which consists of filamentous cells that traverse a continuous cavity between the cortex and the pericycle, so suspending the stele in the centre of the stem. The strobili bear both mega- and microsporangia and produce the largest megaspores of any spore-producing plant. The megaspore contains a large food supply, which enables the female gametophyte to develop independently of external food sources. The microspore develops into an extremely reduced male gametophyte consisting of a prothallial cell and an antheridium, which produces many biflagellate antherozoids. Fertilization relies on a megaspore and microspore being in close proximity and a film of water is needed for the antherozoids to swim to the egg cell of the archegonium. Occasionally fertilization occurs before the megaspore is shed from the parent.