THE MOXOCOTYLEDOXES 



1967 



indeed the critical study of numerous examples has led Hutchinson to 

 emphasize that the Butomaceae and Alismaceae, which are here included 

 in the Helobieae, show marked similarities with the Ranunculaceae and 

 may either have been derived from them or at least may have had a common 

 origin. The main difference between the two groups is due, in his opinion, 

 to the fact that the Monocotyledon families have lost the endosperm in the 

 seeds as an adaptation to an aquatic habitat, a view originally proposed many 

 years ago by Henslow. 



As treated here the Helobieae are regarded as including the follow- 

 ing important families: Alismaceae, Butomaceae, Xaiadaceae, Potamo- 

 getonaceae, Aponogetonaceae, Hydrocharitaceae and Scheuchzeriaceae. 

 Hutchinson recognizes a considerable number of further families due to 

 splitting up several of the above families. Moreover he does not recognize 

 the Helobieae as an order but has created several separate orders to include 

 the families. His method, though undoubtedly having much to commend 

 it, has not been followed here owing to its complexity. 



Of the families mentioned above, we shall consider the Alismaceae in 

 detail, the others we may consider briefly first. 



The Butomaceae are a small family which, according to Bentham and 

 Hooker, is included in the Alismaceae, but w^hich in the works of Engler 

 and of Hutchinson is elevated to family rank. It only includes some four 

 genera most of which are monotypic. The plants occur either in water or 



Fig. 1894.- 



-Biitomus umbel I at us. 

 Inflorescence. 



Flowering Rush. 



2G 



