84 THE JOURNAL OF BOTANY 



graceae, and Limosella, Hydrotrichey etc., among the Scrophu- 

 lariaceae). Finally, an entire family may be aquatic and contain no 

 terrestrial forms (<?.y. Podostemacea?). A family given over wholly 

 to aquatic life may include a number of genera {e. g. Nymphaeaceae 

 and Podostemaceae) or a single genus {e. g. Ceratophylleae and 

 Callitrichaceae). Among the Monocotyledons, on the other hand, 

 we meet with more cases of entire families leading a water life 

 {e. g. Lemnacese, Pontederiaceae, and various families belonging 

 to the Helobieae), but there are fewer instances of individual aquatic 

 genera and species belonging to families which are mainly terres- 

 trial, though these occasionally occur (e. g. Glyceria aquatica of 

 the Gramineae). 



When one genus or species in an otherwise terrestrial family has 

 taken to aquatic life, this may well be held to indicate that the 

 habit is a recent one ; but Avhen a whole family containing a number 

 of genera is found to be hydrophytic, it is hardly possible to avoid the 

 conclusion that the differentiation of the genera has occurred since 

 the adoption of the aquatic habit, which, on this view, must be very 

 ancient. The only other alternative, namely, that all the genem have 

 been evolved in the course of terrestrial life, and that they have all 

 subsequently and independently taken to the water, seems too far- 

 fetched to be considered seriously. A scrutiny of the characters of 

 those aquatic families which contain a number of highly individualized 

 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 Podostemaceae, though not standing 

 so low in the scale of floral evolution, yet appear to be a very 

 old phylum related to the Rosales and Sarraceniales. That is to say, 

 the only Dicotyledonous families which are exclusively aquatic and 

 also contain a number of distinct genera, belong to the more primitive 

 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 man}' 

 ways decidedly Ranalean, and which suggest that the Helobieae 

 represent a branch that took to the water at a very early stage in the 

 evolution of the Monocotyledons, while they still retained features 

 recalling the Ranalean plexus from which they sprang. That they 

 are descended from a geophytic ancestor is suggested by the charac- 

 teristically 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 hypocot3'l of the embryo recalls 

 an ancestor which possessed a hypocotyledonary tuber, resembling 

 that of Erantliis hiemalis, the chief diiference being that in the 

 Helobieae the storage of food in the hypocotjd has been shifted back to 

 a pre-germination stage, owing perhaps to the exigences of aquatic 

 life. It may be recalled in this connexion that tuberous hypocotyls 

 are common among E-anunculaceae with concrescent cotyledons, that 

 is to say, among forms which certain botanists would interpret as 



