xxv] 



HELOBIEAE 



be considered seriously. A scrutiny of the characters of those 

 aquatic families which contain a number of highly individua- 

 lised genera, confirms the notion that such families adopted 

 aquatic life at a relatively early stage in the course of evolution 

 of the Angiosperms. The Nymphaeaceae show characters that 

 are markedly primitive among the Dicotyledons, and the Podo- 

 stemaceae, though not standing so low in 

 the scale of floral evolution, yet appear to 

 be a very old phylum related to the Resales 

 and Sarraceniales. That is to say, the only 

 Dicotyledonous families which are both ex- 

 clusively aquatic and also contain a number 

 of distinct genera, belong to the more primi- 

 tive groups among the Polypetalae, and 

 hence may be regarded as ancient lines 

 which took to the water before they had 

 diverged widely from the ancestral type. 



Among the Helobieae, the Alismaceae 

 are probably nearest to the ancestral stock. 

 This family shows characters which are 

 in many ways decidedly Ranalean, and 

 which^L suggest that the Helobieae re- 

 present a branch that took to the water at a very early stage 

 in the evolution of the Monocotyledons, while they still re- 

 tained features recalling the Ranalean plexus from which they 

 sprang. That they are descended from a geophytic ancestor 

 is suggested by the characteristically abbreviated main axis, 

 which in many cases does not elongate except to form the stalk 

 of the inflorescence. It is also perhaps conceivable that the 

 enlarged hypocotyl of the embryo (Fig. 166) recalls an ancestor 

 which possessed a hypocotyledonary tuber, resembling that of 

 Eranthis hiemalis, the chief difference being that in the Helo- 

 bieae the storage of food in the hypocotyl has been shifted back 

 to a pre-germination stage, owing perhaps to the exigencies of 

 aquatic life 1 . It may be recalled, in this connexion, that tuberous 



1 See pp. 248, 249. 



FIG. 1 66. Ruppia bra- 

 chypus, J. Gay. Longi- 

 tudinal section of fruit. 

 (xi5.) a, cotyledon; 

 b, first leaf following 

 cotyledon ; h, hypo- 

 cotyl; r, primary root. 

 [Raunkiaer, C. (1896).] 



