4 INTRODUCTION [CH. 



among themselves, but characterised, broadly speaking, by a 

 number of features associated with their peculiar mode of life. 

 It is the biological group thus formed which we propose to 

 study in the present book. 



There is good reason to assume that the Angiosperms were 

 originally a terrestrial group and hence that the aquatic Flower- 

 ing Plants existing at the present day can trace back their pedi- 

 gree to terrestrial ancestors. If this be the case, we may interpret 

 the various gradations existing within the hydrophytic group 

 as illustrating a series of stages leading from ordinary terrestrial 

 life to the completest adoption of an aquatic career. At one end 

 of the series we have plants which are normally terrestrial, but 

 which are able to endure occasional submergence, while at the 

 other end we have those wholly aquatic species whose organisa- 

 tion is so closely related to water life that they have lost all 

 capacity for a terrestrial existence. Between these extremes 

 there is an assemblage of forms, bewildering in number and 

 variety. In order to clear one's ideas, it is necessary to make 

 some attempt to classify hydrophytes according to the degree to 

 which they have become committed to water life. It must be 

 realised, however, that, though such a scheme is convenient and 

 helpful in ' pigeon-holeing ' the known facts about aquatics, 

 little stress ought to be laid upon it, except as illustrating the 

 striking variety of form and structure met with among these 

 plants. A classification of aquatics on biological lines is highly 

 artificial, and, since it sometimes places in juxtaposition plants 

 which are quite remote in natural affinity, it has only an indirect 

 bearing on questions of phylogeny. 



The classification of aquatics which forms the second part of 

 the present chapter, is based upon a scheme put forward by 

 Schenck 1 more than thirty years ago, which in its main outlines 

 has never been superseded. But the wider knowledge of the 

 group, which has been acquired since that date, has resulted, as 

 is so often the case, in the blurring of the sharp lines of demar- 

 cation between the individual bionomic classes recognised at an 

 1 Schenck, H. (1885). 



