flowers are either male or female, the former consisting of nothing but a single stamen. The Naiads (_Naias_) form an allied genus, and are slender annual herbs, growing completely submerged in fresh or brackish water. One of them (_N. flexilis_) occurs in lakes at rare intervals along the western edge of the British Isles; and another, _N. marina_, is found living in only one spot in Britain--Hickling Broad in Norfolk; their fossil seeds embedded in old lake deposits show that in former times both were more widely spread than now in Western Europe, and that other species of the genus also occurred. In the Naiads complete reversion to water-pollination is found. When the very simple male flowers shed their pollen, the grains, which are heavy owing to the presence of starch, fall through the water on to the female flowers which are borne below them, or are carried by currents to other flowers. Lastly we come to the Grass-wracks, a small group of submersed marine plants. While some of them, like our little native _Z. nana_, haunt muddy sands between tides, our more familiar species, the common _Z. marina_, is thoroughly marine, growing tall and vigorous among the large Seaweeds down to far below low-water mark (to over 30 feet in the Baltic). The plant has, nevertheless, not yet developed submersed pollination, the pollen-grains rising to the surface, where they are caught by the stigmas of floating female flowers. It follows that the individuals rooted in the deeper water, though growing vigorously, do not mature seed, for the production of which the species has to rely on plants which, at least at low water, are rooted sufficiently near the surface to allow the flowers to rise above it. Could the species achieve submersed pollination, it appears quite capable of colonizing throughout the Laminarian zone, wherever there is a soft substratum for its creeping stems.