CHAPTER XXVI 



THE THEORY OF THE AQUATIC ORIGIN OF 

 MONOCOTYLEDONS 



f I ^HE high proportion of aquatic species among Mono- 

 JL cotyledons, as compared with Dicotyledons, has been 

 noticed in the preceding chapter. This, and other considera- 

 tions, suggested to Professor Henslow his interesting theory of 

 the aquatic origin of Monocotyledons 1 , the broader aspects of 

 which we may now briefly consider. He discusses the number 

 of aquatic families to be found in each of the great groups, and 

 concludes that only 4 per cent, of the Dicotyledonous families 

 are aquatic, as compared with 33 per cent, of the Monocotyle- 

 donous. These figures probably have little absolute value since 

 it is difficult to decide, to begin with, exactly what we are to 

 understand by the expression ' aquatic family ' but they serve 

 a useful purpose in showing how much more numerous 

 aquatics are among Monocotyledons than among Dicotyledons. 

 This is indeed a matter of common observation. It is recorded 2 

 for instance, that in the case of the Bodensee, the plants living 

 in the water or on the margin include forty Monocotyledons 

 and thirty-eight Dicotyledons; this proportion is remarkable 

 when we realise that the total number of species of Monocoty- 

 ledons now existing on the face of the earth, bears to the total of 

 Dicotyledons the ratio 3 , very roughly, of I : 4-5. Henslow's 

 general conclusion, with which most botanists will probably 

 agree, is that marked numerical contrasts of this type "show 

 that there is some decidedly important connexion between an 



1 Henslow, G. (1893) and (1911). It should be recalled that Gardiner, 

 W. (1883) also regards Monocotyledons as essentially aquatic. 



2 Schroter, C. and Kirchner, O. (1902). 



3 The figures from which this ratio is deduced are taken from Coulter, 

 J. M. and Chamberlain, C. J. (1904). 



