This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Order 88. Alismaceae. - Aquatic or marsh plants; leaves mainly radical, net-veined; flowers usually bisexual; perianth 6-partite, wholly or in part petaloid; stamens 6 or more, hypogynous; ovary superior, 3-6 or more, distinct carpels; fruit aggregate of achenes or follicles.
Order 89. Naiadaceae. - Aquatic or marsh plants, herbaceous; leaves floating or submerged, parallel-veined; hermaphrodite, monoecious or dioecious; perianth of 4 scales or none; stamens 1-4, hypogynous; ovary superior; carpels 1-4, apocarpous; ovule 1; fruit I-celled, l-seeded.