706 PROFESSOR F. O. BOWER ON 



Plants that their leaf-architecture in throughout referable to modifications of a 

 branch-system originating 'phyletically in a simple leaf subject to equal dichotomy. 



Postscript. 

 On Theories of the Ultimate Origin of the Leaf. 



The views put forward in this memoir depend upon the way in which the juvenile 

 leaves are regarded. If they are, as is believed, relatively primitive in their char- 

 acters, and the adult leaves are similar forms promoted to higher complexity, then 

 the sequence of leaves in the individual will indicate the probable steps of phyletic 

 advance. Similarly, a comparison of the microphyllous Pteridophytes with the 

 megaphyllous gives a sequence of very similar steps. It seems not unnatural to 

 suggest that along these parallel lines a true indication may be found of how the 

 megaphyllous state originated in descent. Thus it would appear probable that the 

 ultimate origin of the leaf was as a lateral appendage upon the axis, relatively small 

 originally, but capable of enlargement by branching, together with such other steps 

 as are exemplified in the comparisons given above. 



It may be noted that a lateral origin of the leaf accords with the facts of 

 embryology of Vascular Plants. It has been shown by comparison of the embryos 

 of the Pteridophyta that in all adequately examined cases the first segmentation of 

 the zygote stamps the polarity of the embryo, and that the apex of the axis coincides, 

 as nearly as is consistent with the detail of the segmentation, with the centre of the 

 epibasal hemisphere.* Thus in point of fact the position of the axis is defined 

 before the origin of the leaf is apparent. That is so also for the embryos of 

 Gymnosperms and Dicotyledons. But till recently the case of the Monocotyledons, 

 as exemplified by the commonly accepted type of Alisma, seemed to be an exception, 

 for there the cotyledon appears to be terminal. But Coulter and Land t have met 

 this difficulty by showing that Alisma is not a general type of embryogeny for 

 Monocotyledons. In Agapanthus and others there is a peripheral cotyledonarv 

 sheath, surrounding the apex of the axis, which is itself actually central, as in other 

 types of embryos. In the case of Alisma the single cotyledon may then be held to 

 have assumed the terminal position as a consequence of its early and strong 

 development, by a sort of " phyletic slide." If this be so, then the exception of 

 the Monocotyledons as a whole falls away. The "terminal" cotyledon, where it 

 does occur, is not phyletically terminal, but lateral. Thus the conclusion of primary 

 origin of leaves laterally in the embryogeny of Vascular Plants is of general appli- 

 cation, though liable to secondary displacement, as in some Monocotyledons. 



But this conclusion, and this way of regarding the leaf as a member originally 

 microphyllous and progressively asserting itself in descent, is out of harmony with 

 opinions expressed by various recent writers. Thus Campbell has advocated a 



* Land Flora, chap, xlii, p. 670. f Bot. Gaz., 1914, pp. 509-514. 



