326 ORIGIN OF MONOCOTYLEDONS [ch. xxvi 



that the half-dozen purely hydrophytic families of Monocoty- 

 ledons, though they have a v/orld-wide distribution, contain 

 altogether less than two hundred species, whereas the four 

 great world-wide terrestrial families Gramineae, Cyperaceae, 

 Liliaceae and Iridaceae contain ten thousand species. 



As we attempted to show in Chapter xxv, the Helobieae 

 carry every indication of being an ancient group which took 

 to the water very early in the history of the Monocotyledons, 

 and in which the existence of the macropodous embryo has 

 possibly played a considerable part in favouring aquatic life. 

 The Cohort seems in the main monophyletic, though it is 

 conceivable that certain families, therein included, are really 

 offshoots from other Cohorts, which have come by secondary 

 modification to resemble the true Helobieae. 



The two factors that have led to the great development of the 

 Helobieae, and hence to the prevailing impression that there 

 is a strong aquatic tendency among Monocotyledons in general, 

 may be held to be firstly, the long period which has elapsed 

 since the ancestral stock of the Cohort became aquatic^, thus 

 allowing time for its differentiation into a wide variety of forms 

 and secondly, the fortunate provision of an embryo with its 

 food stored in the swollen hypocotyl, which has possibly been 

 one of the chief instruments in determining the remarkable 

 success of the group in aquatic life^. 



1 See pp. 319, 320. 2 See pp. 248, 249. 



