11 CERATOPHYLLACE.E — URTICACE^ 355 



CEEATOPHYLLACEiE 



As in so many submerged plants the leaves are 

 divided into many linear divisions (see p. 25). The 

 flowers are monoecious. The plants float freely, having 

 no roots. 



C. demersum. — The flowers are sessile; male and female 

 in difi'erent whorls, the female generally below. The 

 .males are the more numerous, containing 12-20 anthers, 

 and produce much pollen. The female flowers consist 

 of an ovoid ovary with a filiform sticky pistil. Ludwig ^ 

 points out that though several marine phanerogams 

 flower under water, this is the only European fresh- 

 water species which does so. He has, however, over- 

 looked Najas, four species of which occur in European 

 fresh waters. The anthers terminate in two points, 

 and contain a tannin -like substance which Ludwig 

 suggests protects them from water snails. They also 

 contain air chambers, so that they float on the surface 

 of the water. When they ripen they slightly contract, 

 squeezing the pollen into the water, which it equals in 

 specific gravity. The difi'usion of the pollen is increased 

 by the peculiar movements of the plant itself, which 

 were first noticed by Eodier.^ Sometimes in seven 

 hours an angle of 120° is passed through. The pollen 

 is so plentiful that some of it is almost sure to come in 

 contact with the sticky stigmas. 



UETIGACE^ 

 Leaves rough and often stinging. Flowers unisexual. 



Urtiga 



Wind flowers. Male with 4-lobed perianth and 

 4 stamens. Female perianth with 2 lobes, or if 4, the 



^ " Siisswasserflora, " in Zacharias, Tier- u. Pflanzenwelt d. Susswassers, i. 

 ^ Comptes liendus, Ixxxiv. (1877). 



