xxv] POLYPETALAE 3 1 1 



both with the Caryophyllaceae and Hypericineae 1 , contain 

 the British species Elatine hexandra and E. hydropiper small 

 submerged herbs with minute flowers. Illecebrum (Illecebraceae), 

 again, is so near to the Caryophyllaceae, that it is perhaps best 

 included in this family. 



Polygonum amphibium is an example of an aquatic species 

 belonging to a terrestrial genus and family (Polygonaceae). 

 It is amphibious, but only reaches its optimum growth in water. 



The affinities of the little family Callitrichaceae have been 

 much disputed. Robert Brown 2 , followed by Hooker 3 and 

 Hegelmaier 4 , included it in the Haloragaceae. But it is better 

 related to the Euphorbiaceae ; in this family itself, aquatics 

 are not unknown, e.g. the SaJvinia-like Phyllanthus fluitans 5 . 

 Richard 6 was the first to compare Callitriche with Mercurialis, 

 and more recent work on the relation of its reduced flowers to 

 those of various Euphorbiaceae has rendered it highly pro- 

 bable that he was right 7 . 



The Lythraceae contain a number of marsh plants, such as 

 Lythrum Salicaria, the Water Loosestrife, and also a certain 

 proportion of true aquatics, such as Peplis Portula, with its 

 inconspicuous flowers. 



The Onagraceae include genera occupying very varying 

 habitats; some, such as the Willow Herbs, contain typically 

 terrestrial species, while Ludwigia and Jussiaea are aquatic. A 

 closely related group, generally separated under the name of 

 Haloragaceae, includes Myriophyllum, the Water Milfoil and 

 Trapa, the Bull Nut; Trapa is however sometimes placed in 

 a distinct family, the Hydrocaryaceae 8 . The most problematic 

 genus associated with the Onagraceae is Hippuris. By some 



1 Cambessedes, J. (1829) and Miiller, F. (1877). 



2 Brown, R. (1814) 8 Hooker, J. D. (1847). 

 4 Hegelmaier, F. (1864). 6 Spruce, R. (1908). 



Richard, L. C. (1808). 7 Baillon, H. (1858) and Lebel, E. (1863). 



8 The distinctness of Trapa from the Onagraceae has recently been 

 emphasized by Tackholm, G. (1914) and (1915) on the ground of its 

 embryo-sac characters. 



