Manual of the New Zealand Flora/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 (trabeculæ), 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.
1. ISOETES, Linn.
Characters of the order.
Widely distributed in most temperate and tropical regions. Over 60 species are admitted in the most recent enumeration, but many of them are very imperfectly known. The two found in New Zealand are endemic.
| Macrospores tubercled | 1. I. Kirkii. |
| Macrospores smooth | 2. I. alpinus. |
1. I. Kirkii, A. Braun in Berl. Monatber., July, 1869.—Permanently submerged. Stem short, tuberous, deeply 3-lobed; roots numerous. Leaves 10–30, very slender, 3–12 in. long, about 1/20 in. diam., tapering to a fine point, pale-green, diaphanous, usually with a few stomata, but with no accessory bast-bundles. Sporangia rather small, broadly oblong or almost globose, about ⅛ in. long; indusium complete. Macrospores rather small, chalky-white, beset all over with small unequal tubercles. Microspores very minute, trigonous, most minutely punctate.—Kirk in Trans. N.Z. Inst. ii. (1870) 107, t. 7; Bak. Fern Allies, 127; Thoms. N.Z. Ferns, 109.
North Island: Wairua River, A. Thompson! lakes in the Middle Waikato Basin, abundant, Kirk! T.F.C.; Lake Rotokakahi, Kirk! Lake Taupo, C. J. Norton! South Island: Small lakes near Lake Tekapo, T.F.C. Sea-level to 2500 ft.
Best distinguished from the following species by its smaller size and more slender habit, fewer leaves, and particularly by the conspicuously tubercled macrospores.
2. I. alpinus, T. Kirk in Trans. N.Z. Inst. vii. (1875) 377, t. 25.—Permanently submerged; much larger and stouter than I. Kirkii. Stem stout, deeply 3-lobed. Leaves very numerous, 30–70, 6–18 in. long or even more, 1/15–1/10 in. broad at the middle, much dilated at the base, gradually tapering to the apex, dark-green, diaphanous, usually with a few stomata, but with no accessory bast-bundles. Sporangia oblong, ⅙–⅕ in. long; indusium complete. Macrospores greyish-white, smooth or very indistinctly tubercled.—Bak. Fern Allies, 127; Thoms. N.Z. Ferns, 109. (?)I. multiangularis, Col. in Trans. N.Z. Inst. xxii. (1890) 449.
North Island: Lake Taupo, C. J. Norton. South Island: Not uncommon in lakes in mountain districts, from Nelson to the south of Otago. 1200–3000 ft.