Isoëtales

an order of sporophytic plants of the division Lycopodiophyta; it includes the single family Isoëtaceae. The plants are herbaceous perennials capable of secondary growth. The stem is bulbously thickened and short. Its base is usually divided by furrows into two or three lobes on which develop many thin, forking roots. Above is a rosette of spiraling subulate leaves. Inside each leaf run four air canals; on the inner side, at the point where the widened section becomes subulate, is a glumaceous ligule.

The Isoëtales are heterosporous plants. Sporangia form between the base of the leaf and the ligule. They are often covered by a membranous growth (velum). The outer leaves bear large megasporangia with megaspores (160–240 in each), and the inner leaves bear small microsporangia with microspores (up to 1 million). As they grow, the microspores give rise to the male gametophyte, in whose antheridia develop several multiflagellate spermatozoids. From the megaspore is formed the female gametophyte. From the fertilized egg cells of the archegonia grows a new plant, the sporophyte. The Isoëtales have two extant genera, Isoëtes and Stylites. The latter has a single species, found in Peru. The order also has several extinct genera.