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.
The habitat of this species is brackish water. The leaves are opposite, thread-like. The anthers are 2-celled, the anther-stalks very short. The stigma is large and wavy on the margin. The style is very short. The achenes are 5-6, more or less stalkless. The nuts are close together, crenulate on the back. The plant flowers between May and August, and is a herbaceous annual.
Naias flex His, Rostkov & Schmidt. - The habitat of this plant is deep lakes. The stem is branched, brittle, slender. The leaves are very slender, narrow, ternate, 3 in a whorl, or opposite, transparent, with very small distant teeth, or entire, veined. The sheaths are fringed with hairs, or have very small teeth. The flowers are solitary or 2-3. The anther is i-locular. The style is short. The ovary is solitary, stalkless, in the axils. The plant is floating, flowering in August and September, and is a herbaceous perennial.
Naias marina, L. ( = N. major, All.). - This is a very rare plant. The stems are repeatedly forked into 2 branches from near the base, with a few teeth. The leaves are opposite, or in whorls of 3, linear, with numerous, large, spinous teeth, the back and upper internodes also bearing spines, and with coarse teeth. The sheaths are round and more or less entire. The flowers are solitary. The male flower is enclosed in a spathe. The anther is 4-locular. The drupe is ellipsoid or ovoid, purplish, succulent, and narrowed above into a stout style which does not fall. The plant is 3-9 in. in height, flowering in July, and is a herbaceous perennial.
This has been introduced into Britain in a canal at Reddish, Lancashire, in Egyptian cotton, from a mill which pours out waste hot water. The stem is branched throughout, with numerous, simple, lateral branches. The leaves are tufted, narrow, linear, with many small, spinous teeth. The sheaths are toothed and auricled. The male flowers are stalked, with 4-locular anthers. There are 2 stigmas. The fruit is either solitary or in groups of 2-4, and is narrowly ellipsoid. The plant is floating, flowerv ing between July and September, and is a herbaceous annual.