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CHAPTER XXV 



THE AFFINITIES OF WATER PLANTS AND THEIR 



SYSTEMATIC DISTRIBUTION AMONG THE 



ANGIOSPERMS 



(i) THE AFFINITIES OF CERTAIN AQUATIC ANGIOSPERMS 



IT is generally recognised that the primaeval forms of 

 vegetable life were probably aquatic, and that it is only in 

 the highly evolved group of Seed Plants that a terrestrial habit 

 has become firmly established. It follows that any aquatics met 

 with among the higher plants must be regarded as descendants 

 of terrestrial ancestors, which have reverted in some degree 

 to the hydrophytic habits of their remote forbears. That this 

 view is tenable, and that the Aquatic Angiosperms cannot trace 

 their ancestry in an unbroken aquatic line from some far-away 

 algal progenitor, is demonstrated by the fact that their floral 

 organs, in the vast majority of cases, belong to a decidedly 

 terrestrial type 1 . 



Before discussing any significance which may be attributed 

 to the systematic distribution of aquatics among the families 

 and genera of terrestrial Angiosperms, it will be necessary 

 briefly to review the natural affinities of various members of this 

 biological group affinities which are still in some cases deci- 

 dedly problematical. The present writer accepts the theory that 

 the Ranalean plexus includes the most primitive forms among 

 the living Angiosperms 2 , and also the view that from this plexus 

 the Monocotyledons have been derived 3 ; these theories provide 

 the basis for the general order in which the plants are dealt with 

 in this chapter, and they also form the bed-rock for the dis- 

 cussion arising out of the facts enumerated. 



1 See Chapter xvm. 2 Arber, E. A. N. and Parkin, J. (1907). 



3 Sargant, E. (1908) and earlier papers. 



