314 AFFINITIES [ch. 



semble in polycarpy, polyandry and the insertion of the ovules^. 

 The scattered arrangement of the ovules on the carpellary wall 

 of Butomus and Vallisneria is similar to that observed in certain 

 Nymphaeaceae, while coalescence and epigyny occur in both 

 Hydrocharitaceae and Nymphaeaceae^. The view that these 

 resemblances are not indicative of affinity, and that the develop- 

 ment of a similar type of flower in the two families is mere coin- 

 cidence^, seems to the present writer to have little to support it, 

 except the fact that the flowers of the Ranunculaceae are gene- 

 rally more or less acyclic, while those of the Alismaceae have 

 the parts whorled. This argument scarcely seems to carry much 

 weight, when it is recalled that certain genera which are un- 

 doubtedly members of the Ranunculaceae, e.g. Aquilegia^ have 

 flowers which are verticillate throughout. 



The Helobieae as a whole appear to be more nearly related 

 to the Spathiflorae (Araceae and Lemnaceae) than to any other 

 Cohort of Monocotyledons^ the Aponogetonaceae forming, 

 in some respects, a link between the two Cohorts. This family 

 recalls the Araceae in its sympodial growth and tuberous stem, 

 its laticiferous tissue and its flower spike with a fleshy axis. In 

 the perforation of the leaves, Aponogeton jenestralis may be 

 compared with Monstera^ . But the Aponogetonaceae show, in 

 addition, certain distinctively Helobian characters, which have 

 led to their association with the Alismaceae, Juncaginaceae and 

 Potamogetonaceae ^. 



The Potamogetonaceae and Naiadaceae seem to form a co- 

 herent group, while their affinity with the other members of the 

 Cohort is by no means a close one. The Potamogetonaceae share 

 with the Hydrocharitaceae one curious little character, which 

 may be of systematic i-mportance, the occurrence, namely, of 

 peculiar teeth at the edge of the leaf, formed from elongated cells 



1 Buchenau, F. (1903^). ^ Schaffner, J. H. (1904). 



3 Rendle, A. B. (1904). 



* Hegelmaier, F. (1868) and Engler, A. (1892). 



^ Sergueeff, M. (1907). 



^ Planchon, J. E. (1844) and Krause, K. and Engler, A. (1906). 



