Herbs, often aquatic, rarely undershrubs. Leaves opposite, alternate, or whorled, when submerged often pectinately pinnatifid; stipules wanting. Flowers hermaphrodite or unisexual, always small and often incomplete. Calyx-tube adnate to the ovary; lobes 2, 4, or wanting. Petals 2, 4, or wanting, valvate or slightly imbricate. Stamens 2 or 4–8, rarely 1 or 3, large, epigynous; filaments short, filiform; anthers 2-celled. Ovary inferior, compressed, angled or ribbed, rarely 2–4-winged. 2- or 4-celled, rarely 3-celled; styles 1–4, distinct; stigmas papillose or plumose; ovules as many as the styles, pendulous, anatropous. Fruit small, dry or succulent, 1–4-celled, indehiscent or separating into 1–4 indehiscent carpels. Seeds solitary in the cells, pendulous; albumen fleshy, usually copious; embryo cylindrical, axile.

A small order of mostly inconspicuous plants, many of them water-weeds. Genera 8 or 9; species from 80 to 90. I have followed Hooker and Bentham in keeping Callitriche in this order, but it must be admitted that it has equal claims to be placed among the Monochlamydeœ. Of the 4 New Zealand genera, Haloragis is mainly Australian, but extends northwards as far as Japan; Myriophyllum and Callitriche are almost of world-wide occurrence; while Gunnera belongs to the south temperate zone.

Terrestrial. Calyx 4-lobed. Stamens 4–8. Petals valvate. Fruit nut-like, undivided	1. Haloragis.
Aquatic. Calyx-lobes obscure. Stamens 4–8. Petals imbricate. Fruit separating into 2–4 nut-like carpels	2. Myriophtllum.
Subaquatic or terrestrial. Stamens usually 2. Fruit a 1-seeded drupe	3. Gunnera.
Aquatic or subaquatic. Stamen 1. Styles 2. Seeds 4	4. Callitriche.