Hydroplzytes and hemi-hydrophytes (aquatic plants).Of marine hydrophytes, there are, in this country, only the grass-wracks (Zostera marina and Z.
Very few aquatic plants are pollinated under water, but this is wellknown to occur in species of Zostera and of Naias.
Harbours and shallows covered with Zostera are likewise favourite haunts of this species, although the water may be brackish.
Porphyra laciniata, the edible laver; Codium tomentosum, a coarse species; Padina pavonia, common in shallow water; Ulva latissima; Haliseris polypodioides; Sargassum bacciferum; the well-known gulf weed, probably transported from the Atlantic; Zostera marina, forming dense beds in muddy bays; the roots are cast up by storms and are valuable to dress the fields.
In these the pollen floats on the surface and reaches the stigmas of the female flowers as in Callitriche, Ruppia, Zostera, Elodea.
Sea-aster flourishes in the Wadden of Friesland and Groningen, the Dollart and the Zeeland estuaries, giving place nearer the shore to sandspurry (Spergularia), or sea-poa or floating meadow grass (Glyceria maritima), which grows up to the dikes, and affords pasture for cattle and sheep. Along the coast of Overysel and in the Biesbosch lake club-rush, or scirpus, is planted in considerable quantities for the hat-making industry, and common sea-wrack (Zostera marina) is found in large patches in the northern half of the Zuider Zee, where it is gathered for trade purposes during the months of June, July and August.
A bed of Zostera near the level of low water is, however, on the British coast a favourite collecting ground for the smaller red and brown epiphytes.
The entrances to the inner lagoons of the Limfjord are naturally blocked against the immigration of flatfish by dense growths of sea-grass (Zostera), although the outer lagoons are annually invaded by large numbers of small plaice from the North Sea.
A well-known example of a permanently sessile form is Lucernaria, common on the Atlantic coasts of Europe, especially in Zostera-beds, attached to the weed.
In some aquatics, as Zostera, Zannichellia, Naias, &c., only one covering exists.