

xxv] NAIAS 315 



with thickened walls 1 . The Potamogetonaceae, like the Apono- 

 getonaceae, show certain features which suggest the Araceae. 

 Zostera, in particular, was actually included among the Aroids 

 by de Jussieu 2 , while, nearly a century later, Engler 3 suggested 

 that the carpels and anthers of this genus might possibly each 

 represent a male or female flower, the arrangement thus being 

 comparable with that prevailing in the Aroid Spathicarpa. 



The position of the Naiadaceae is obscure, owing to the 

 difficulty of interpreting the extremely simple flower. Rendle, 

 in his authoritative work on Naias, regards it as an "appa- 

 rently primitive type of Monocotyledon 4 ." Such a view is of 

 course entirely irreconcilable with the belief that the Mono- 

 cotyledons are derived from some early member of the Ranalean 

 plexus, and that the primitive Angiospermous flower was of the 

 ' Eu-anthostrobilus ' type 5 , with a petaloid perianth of numerous 

 members, and numerous free stamens and carpels. On this view 

 Naiasmust be interpreted as a highly reduced form, representing 

 perhaps the ultimate term of reduction in the Potamogetona- 

 ceae series. The female flower consists of a single ovule, around 

 which a carpellary wall and integuments grow up in a rather 

 belated fashion. The flower is sometimes naked, but sometimes 

 surrounded by a membranous bottle-shaped envelope. The 

 male flower consists of a single stamen, enclosed in most cases 

 in two such envelopes, but sometimes in one only. According 

 to Rendle, the outer envelope of the male flower, and the corre- 

 sponding envelope which occasionally invests the female flower, 

 are of the nature of spathes, comparable with the spathes 

 occurring in other submerged water plants, e.g. Hydrilla, and 

 with the membranous cup enclosing the female flowers of 

 Zannichellia. The probabilities are perhaps in favour of this 

 interpretation, but it is more difficult to agree with Rendle's 

 explanation of the inner envelope of the male flower, which 

 he regards as a perianth. The present writer would like to 



1 Ascherson, P. and Graebner, P. (1907) ; see also p. 133. 



2 Jussieu, A. L. de (i 789). 3 Engler, A. (i 879). 



4 Rendle, A. B. (1899). 5 Arber, E. A. N. and Parkin, J. (1907). 



