Manual of the New Zealand Flora.
Order XCVII. IsoetaceÆ
Order XCVII. IsoetaceÆ.
Aquatic or amphibious perennials, often entirely submerged, rarely terrestrial. Stem (corm) short and tuberous, of firm texture, 2–3-lobed; roots numerous, rather fleshy, dichotomously forked, springing from the furrows of the stem. Leaves many, forming a dense tuft at the top of the stem, linear or filiform, flat in front, rounded at the back, dilated and sheathing at the base, always furnished with 4 longitudinal air-channels divided by transverse partitions, and with a single central vascular bundle; stomata. present, or absent in those species which are permanently submerged. Sporangia large, membranous, placed in a hollow (fovea) of the dilated base of the leaf and sometimes partly concealed by the membranous margin (velum or indusium) of the fovea, 1-celled, but often imperfectly divided by rods or plates of tissue (trabeculse), those of the outer leaves containing macrospores, those of the inner leaves microspores. Immediately above the fovea is a flattened membranous plate called the ligule. Macrospores large, globose, usually chalky - white, with three ribs radiating from the apex. Microspores very minute, trigonous.
A very isolated family, consisting of the single genus Isoetes, allied on the one hand to the Lycopods and on the other to ferns. In germination a male prothallium. with a single antheridium containing spermatozoids is developed within the microspore, the spermatozoids being set free by the bursting of the coats of the microspores. The macrospores in a similar manner each produce a female prothallium bearing 2 or 3 archegonia, which are ultimately exposed by the splitting of the macrospore along the three ridges. Fertilisation then takes place in the same way as in ferns.