[58] 



CHAPTER V 



THE LIFE-HISTORY OF THE POTAMOGETONA- 

 CEAE OF FRESH WATERS 1 



POTAMOGETON, the central genus of the Potamoge- 

 tonaceae, includes the very numerous Pondweeds, so 

 common in temperate waters, and is the richest in species of all 

 our native aquatic genera. The Pondweeds are an exceedingly 

 difficult group from the point of view of the student of system- 

 atic botany, as the numerous species can, in many cases, only 

 be discriminated as the result of much experience. A character 

 which increases the difficulty of identifying them is the capa- 

 city for variation in form shown by one and the same individual. 

 The present writer took a typical shoot of Potamogeton-perfoliatus 

 from the Cam in July, and kept it floating in a rain-water tub. 

 By October I most of the large perfoliate leaves had decayed 

 and those on the new shoots were so much narrower and less 

 perfoliate as to make it difficult to believe that they belonged 

 to the same species (Fig. 36). This power of variation in 

 leaf-form within one individual is a well-known feature of 

 P. perfoliatus. It has been recorded that an isolated plant in a 

 newly-dug clay-pit, observed during several years, changed so 

 much in the shape, colour and texture of the leaves as to give 

 rise to the idea that all the British forms of the species which 

 have been described, may possibly be mere states and not 

 variations 2 . 



The most obvious difference between the Potamogetons and 

 the water plants hitherto considered, lies in the extreme com- 

 plexity of the shoot systems of the Pondweeds. The rhizomes 



1 The marine Potamogetonaceae are considered in Chapter x. 



2 Fryer, A., Bennett, A. and Evans, A. H. (1898-1915). This account 

 of the British Potamogetons is of the first importance. 



