204 SOME BRITISH PLANT GROUPS 
numerous Pondweeds which grow in our lakes and 
rivers—plants with leaves broad and floating, or 
narrow and submerged, and inconspicuous flowers 
which rise above the water and are pollinated by the 
wind. Next we find several narrow-leaved Pondweeds 
which grow in brackish pools; and with them are 
some allies, the Tassel Pondweed (Ruppia) and 
Horned Pondweed (Zannichellia), with more reduced 
flowers and often a more nearly marine habitat, as 
they sometimes mix with Seaweeds on the open 
shores of estuaries; in these plants we find the stages 
of a most interesting return to the archaic method of 
water-pollination, so long discarded by the great mass 
of the Seed Plants. In the flower of Ruppia, which 
consists merely of two stamens and four carpels with- 
out corolla or calyx, the pollen is liberated under 
water, and, being light, rises to the surface; older 
flowers have already, by growth of the flower-stalk, 
reached the surface, and they become pollinated by 
the floating grains. In Zannichellia the process is in 
general similar, save that the 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 com- 
pletely submerged in fresh or brackish water. One of 
them (N. flevilis) 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 rever- . 
