THE LEAP. 



229 



formed leaves consist solely of the leaf-base, as in 

 Cycads. 



These facts suggest that the sheathing base repre- 

 sents the most primitive part of the leaf, and that 

 petiole and lamina are a later acquisition. Consequently 

 those groups of plants in which the sheathing base is 

 the most conspicuous part of the entire foliage-leaf, and 

 in which petiole and lamina are not so frequently 

 differentiated as distinct areas, must be regarded as 

 possessing the most primitive type of vegetative stem. 

 As the majority of Monocotyledons exhibit this type 

 they must inevitably be held to be a more ancient 

 group of plants than the Dicotyledons, in which the 

 leaf -conformation is much more highly differentiated. 

 The view that the sheathing leaf -base (and all that it 

 involves) is the result of the geophilous habit of so many 

 Monocotyledons, suggested by Miss Sargant, would ap- 

 pear to be refuted by the fact that the Palms, so primitive 

 in their floral conformation, possess the same sheathing 

 leaf-bases,* although, as tall trees, they represent the 

 very antitheses of geophilous plants. The Palms also, 

 so well-developed in all their parts, constitute a refuta- 

 tion of the modern view that Monocotyledons have been 

 derived from the Dicotyledonous type by reduction. 

 A further important point is this : that the occurrence, 

 although rarely, of dichotomy of the stem as a normal 

 feature in Monocotyledons and never in Dicotyledons 

 shows the former class to be more primitive than the 

 latter, and that the Palms, in which it alone occurs, 

 are, consequently, more primitive than other members 

 of the class. 



These then are our clues as to what was the original 

 type of phyllotaxis in the vegetative portion of the 

 axis ; for it is probable that in the far-back (fern-like r) 

 ancestors of Angiosperms there was no great distinction 

 between sporophylls and vegetative leaves either as 



* Here, however, the leaf is differentiated into petiole and lamina, indicat- 

 ing that the Palms have subsequently and independently evolved the higher 

 type of leaf-conformation. 



