MONOCOTYLEDONS 195 



simple flowers tlie simplest are found in the Naiad 

 Family — ^the Naiadese, in the particular genus Naias, 

 composed of submerged aquatic plants represented 

 in all parts of the world; the flowers are unisexual, 

 the male consisting of a single stamen in two cuplike 

 envelopes, or bracts; the female of a single carpel, 

 without a stalk, sitting in the sheathing base of the leaf. 

 The Naiads occur in fresh or brackish water; the leaves, 

 according to species, may be linear, with toothed or 

 entire margins. They are arranged oppositely on the 

 stem. These plants are not common in Britain. The 

 North American species, Naias fiexilis, has been found 

 in Perthshire, Skye, and Connemara; the Holly-leaved 

 Naiad, N. marina, common in tropical and in some 

 temperate regions of the Old World, is recorded as 

 having been found in Hickling Broad, Norfolk. 



The Grass Wrack, Zostera marina, is common on 

 sand and mud flats near the edge of the sea in temperate 

 regions; it is found at or below low- water mark, even at 

 a depth of 30 or more feet, and is frequently thrown 

 ashore in considerable quantities by the tide. The 

 stem is creeping and roots in the mud or sand; the 

 leaves are grasslike, long,, and arranged alternately; 

 they often reach a length of several feet. But although 

 Zostera flourishes in the sea, it is not a seaweed — i.e., an 

 Alga — and although its leaves are grasslike, it is not a 

 grass. The simple flowers occur in a flat spike enclosed 

 in the sheath at the base of a foliage leaf; they consist 

 of a few stalkless anthers and three or four carpels. 

 The pollen of this plant is peculiar. It is not found in 

 the form of round grains, but in long thread-like fila- 

 ments; these are of the same specific gravity as the water 



