394 BRITISH FLOWERING PLANTS chap. 



ISTAI ADAGES 



Floating or submerged plants, with very simple 

 flowers. Pollen without extine. 



ZoSTERA (Sea-grass) 



Marine grass-like herbs, with flowers enclosed in a 

 sheath near the base of the leaves, and always submerged. 

 The pollen forms long slender thread-like tubes, the 

 so-called confer void pollen from its resemblance to an 

 algal filament. The thin hair-like styles are exserted 

 from the spathe to catch the pollen. We have two 

 species — Z. marina with ribbed seeds, common in 

 muddy and sandy estuaries, and Z. nana with smooth 

 seeds, a much smaller and rare plant. 



Najas 



Small submerged herbs growing in fresh or brackish 

 water, with a slender stem and small opposite leaves 

 with a narrow blade and basal sheath. The flowers are 

 unisexual. The male flower consists of an anther, closely 

 surrounded by a thin membranous sac-like perianth. The 

 pollen is spherical or oblong. The female flower is a 

 more or less ellipsoidal ovary, produced into a narrow 

 style with usually three stigmas. 



We have two species, both very local. N. marina 

 is dioecious, and N. Jlexilis monoecious. 



N. marina is almost world-wide ; but in Britain 

 occurs only in one or two localities in the Norfolk 

 Broads, though in prehistoric times it was more widely 

 distributed. 



N. jlexilis is a North American species which has 

 been found in several localities in Ireland, Scotland, and 

 on the Continent. 



Mr. Clement Eeid's discovery of fossil seeds in recent 

 beds indicates that these species were once more 

 generally distributed in Britain, and also the presence 

 of two other species — N. minor, which is now 

 widely spread in Europe and Asia, and N. graminea, 



