THE EVOLUTION OF PLANTS 



227 



dicotyledons, by a fusion of the two cotyledons into one. l 

 On this basis the single seed-leaf of monocotyledons is 

 interpreted as homologous to the two seed-leaves of di- 

 cotyledons. The evidence supporting this suggestion is 

 derived largely from a study of the anatomy of monocoty- 

 ledonous seedlings. "The young epicotyl of monocotyle- 

 donous seedlings contains a single ring of collateral bundles 

 which may even show traces of cambium, much resembling 



FIG. 107. A-B, embryos of a "dicotyledonous monocotyledon," A, 

 longitudinal section through an embryo of Tamus communis; B, Tamns 

 communis, entire (A and B enlarged after Solms-Laubach.). C-G, 

 embryos of "monocotyledonus dicotyledons;" C, D, Dioscorea bonariensis, 

 enlarged (after Beccari); E, Trapa natans, the water chestnut X Vi 

 (after Barneoud); F, Pinguicula vulgaris; G, Pinguicula caudata. (F and 

 G after Dickson, both greatly enlarged.) 



dicotyledons." Professor Jeffrey has also called attention 

 to evidence that the anatomy of the stem of the hypotheti- 

 cal ancestor of the Angiosperms was exogenous (dicotyle- 

 donous). 



Miss Sargant has further pointed out that the few 

 dicotyledons which possess but one seed-leaf (pseudo- 

 monocots) are widely distributed through the dicotyle- 

 donous families, from Ranunculaceae to Umbilliferae, 



1 Annals of Botany 17: 1-88. Jan., 1903; Botanical Gazette 37: 325- 

 345. May, 1904, and other papers. 



