THE BOOK OF NATURE STUDY 



has been restricted to those likely to be encountered, or specially 

 interesting and valuable inhabitants of the aquarium. 



Included by Bentham and Hooker in the Naiadaceae, but often 

 placed in a separate family, are the pondweeds or Potamogetons, 



common forms found alike in 

 ponds and lakes and in canals or 

 slow-flowing streams. There are 

 a large number of British species 

 not easy to distinguish from one 

 another. The common forms are 

 useful plants for the aquarium. 

 All have rootstocks, which for 

 the most part require to be 

 planted in soil at the bottom of 

 the aquarium. The leaves are 

 opposite or alternate, submerged 

 or floating, usually ribbon-shaped 

 with sheathing bases. The flowers 

 are small, and are placed in ses- 

 sile heads or spikes which pro- 

 trude above the surface of the 

 water. The most primitive form 

 is P. natans, which is perhaps 

 the commonest. It has leathery 

 floating leaves and filamentous 

 submerged ones. It dies down in 

 winter to the root stock. Curly 

 pondweed (P. crispus, p. 91) has 

 all the leaves submerged; they are 

 narrow and waved at the mar- 

 gins. This form produces in- 

 teresting winter buds with broad 

 leaves. An attempt should be 

 made to keep these through the 

 winter, or to take them from the 

 ponds in spring in order to note 



FlG. 41. Zostera marina, showing the 

 creeping rootstock, the fibrous roots, 

 and the long slender leaves which float 

 out in the water. 



their unfolding. In perfoliate 

 pond weed (P. perfoliatiis) the 



