AQUATIC ANGIOSPERMS 85 



supplying indications of the characters of the original Monocotyle- 

 donous stock. 



The idea that the Helobiese are descended from a very ancient 

 group of Angiosperms and have inhabited the water for a correspond- 

 ingly long period, is ratified by the fact that this series consists of a 

 whole plexus of related families, some of which have departed widely 

 from the original type ; it contains forms as far asunder, for instance, 

 as Alisma with its many Ranalean features and Naias which repre- 

 sents the very acme of floral reduction. 



The fact that the Nymphseacese and the related Ceratopli3^11ea3 on 

 the one hand, and the Helobiese on the other, have taken to aquatic 

 life with such conspicuous success suggests that the original Ranalean 

 stock from which they both sprang ma^^ have been particularly well 

 adapted to water life. In the Ranunculacese the tendency to aquatic 

 habits in the case of the genus Ranunculus is obvious; besides the 

 definitely aquatic sub-genus Bah^achium^ the Buttercups include 

 a number of forms, such as It. sceleratus and R. jiamnnda, which arc 

 capable both of land and water life. The singularly slight difference 

 in general anatomy between the terrestrial and aquatic species of 

 Ranunculus, suggests that the land forms are of a type which does 

 not require great changes of structure in order to succeed in water life. 



It is a remarkable fact that the Sympetalae — the most highly 

 evolved group of Angiosperms — has produced no entirely aquatic 

 family, nor any single aquatic species which has become so far 

 adapted to water life as to have acquired submerged hydrophilous 

 pollination. The very large family of the Compositae, which may 

 perhaps be classed as the ultimate term of the Sympetalous series,. 

 contains apparently less than half-a-dozen aquatic members. Exactly 

 the same is true of all the earlier cohorts of Engler's Archichlamydese,, 

 which, on the present writer's view, represent the more advanced and 

 reduced forms of the series. The families which are generally known 

 as Polypetalse (the later cohorts of Engler's Archichlamydese) and 

 which, on this view, include all the more primitive Dicotyledons, are 

 markedly richer in aquatic types. It would hardly be going too far 

 to say that independent aquatic families are chiefly characteristic of 

 the Ranalean plexus, and of its derivatives — both Dicotyledonous and 

 Monocotyledonous — while among the more advanced Polypetalse and 

 the Sympetalae, the sporadic occurrence of aquatic types and their 

 close relation to terrestrial forms, indicate that the water-habit has 

 been acquired comparatively recently. It is always possible that those 

 individual genera and species among the Sympetalae which are aquatic 

 at the present day, may each, in some future age, be represented by 

 an entire aquatic family; for such groups as the Helobieae, Nym- 

 phaeaceae and Podostemaceae may owe their richness in genera and 

 species partly to their ancient birth and to the length of time that 

 has elapsed since they took to the water. But, on the other hand, a 

 member of the Sympetalae embarking at the present day upon an 

 aquatic career, may possibly be handicapped, as a potential ancestor, 

 by the high degree of specialization it has attained in its previous 

 terrestrial life. The members of the primaeval Ranalean plexus may 

 have possessed a greater plasticity in correlation with their lower 



