II NAIADACE.i; 395 



common in the tropics of the old world, and finding its 

 present northern limit in the Mediterranean area.' 



Zannichellia 



Z. palustris (Horned Pondweed) is more or less 

 common in fresh or brackish water, in temperate and 

 tropical regions all over the world. The plants are sub- 

 merged. The long-stalked male flowers consist merely 

 of one or two stamens ; the female, of single free or 

 shortly-stalked carpels, collected in groups of four or 

 fewer, and surrounded by a cup-like or entire spathe. 

 The pollen is spherical. 



RUPPIA 



R. maritima is widely distributed in salt and brackish 

 water in the temperate and tropical zones. The simple 

 hermaphrodite flowers are enclosed in the broad swollen 

 sheaths of the two uppermost foliage leaves up to the 

 time of flowering, but are then raised to the surface 

 by elongation of the peduncle, which lengthens still 

 further after fertilisation, and often becomes spirally 

 coiled. The pollen is bow-shaped and floated by the 

 water to the stigmas. 



PoTAMOGETON (Pondweed) 



Protogynous wind flowers. Parts of the flower in 

 fours. The flowers are borne in spikes which rise 

 above the water. Some botanists make fifty species, 

 of which no less than twenty-one are British. Others, 

 however, reduce the number considerably ; Bentham 

 reduces ours to nine — probably too low an estimate. 



In P. crispus shoots with short leaves quite unlike 

 the usual form are developed in autumn at the ends 

 of some of the branches, detach themselves from the 

 mother plant, sink to the bottom, and root in the 

 mud. 



P. natans. — This species afi"ords a good illustration 

 of the way in which the chlorophyll grains adapt 



1 Rendle, "British Species of Najas," in Journal of Botany, 1900. 



