2 26 HEREDITY AND EVOLUTION IN PLANTS 



line with the suspensor, finally making the cotyledon ter- 

 minal." This he calls "the real interpretation of a mono- 

 cotyledonous embryo." Henslow further inferred a very 

 early origin of monocotyledons from dicotyledons, from 

 the fact that so many of their orders contain very few gen- 

 nera and monotypic groups, for groups of plants or animals 

 with few members, are regarded, in general, as survivals, 

 representing a lost ancestry. He recorded voluminous 

 observations in support of his theory, and, among other 

 evidence, called attention to " Dicotyledonous monocoty- 

 ledons "such as Tamus communis (black bryony) , a tuberous 

 rhizomed species of the Yam family, where the first foliage 

 leaf, situated exactly opposite the cotyledon, is interpreted 

 (with Dutrochet) as a second cotyledon; and to "Mono- 

 cotyledonous dicotyledons," especially among aquatic 

 species such as the water-chestnut {Trapa natans), where 

 one cotyledon is arrested in its development. Other illus- 

 trations, not mentioned by Henslow, include such forms 

 as Dioscorea bonariensis and Pinguicula vulgaris (Fig. 107). 

 Ranunculus Ficaria is not an aquatic, but it flourishes 

 by the waterside, and is regarded by Henslow as descended 

 from an aquatic form. About one-third of the orders of 

 of monocotyledons are aquatic, as compared to only 4 

 per cent, in dicotyledons, and the monocotyledonous dicoty- 

 ledons are all aquatic. The final conclusion of Henslow 

 is, "that endogens [monocotyledons] have in the first place 

 descended from very early types of exogens [dicotyledons] 

 . . . ; and that, secondly, the more immediate cause of 

 their origin was an aquatic habit of life assumed by certain 

 primitive exogenous plants." 



Miss Ethel Sargant has more recently elaborated the 

 hypothesis of the derivation of monocotyledons from 



