PLANT MORPHOLOGY 71 



depends upon the leaf-formation, but the first formation of which 

 precedes that of the leaves." Unger also in his botanical letters to 

 a friend (no. viii), described how "The first endeavor is directed 

 towards the building up with cell-elements of an axis;" " those 

 variously formed supplementary organs which are termed leaves" 

 originate laterally upon it; and he concludes that "we may therefore 

 say with perfect justice that the plant ... is, as regards form, essen- 

 tially a system of axes." Naegeli contemplated a somewhat similar 

 origin of the leafy shoot, as an alternative possibility; in fact, that 

 the apex of a sporogonium-like body elongated directly into that of 

 the leafy stem: in which case the axis would be the persistent and 

 prominent part, and the leaves be from the first subsidiary and 

 lateral appendages. In my theory of the strobilus in archegoniate 

 plants the central idea was similar to this ; it may be briefly stated 

 thus: There seems good reason to hold that a body of radial con- 

 struction, having distinction of apex and base and localized apical 

 growth as its leading characters, existed prior to the development 

 of lateral appendages in the sporophyte: for such a body is seen in 

 certain bryophyte sporogonia, while the prior existence of the axis, 

 and lateral origin of the appendages upon it, is general for normal 

 leafy shoots. The view thus put forward is indeed the mere reading 

 of the story of the evolution of leaves in terms of their normal indi- 

 vidual development. I have recently shown that all pteridophyte 

 shoots may be regarded as derivatives from the radial strobiloid 

 type, with relatively small leaves, which would thus have come into 

 existence. 



It is natural to look to the pteridophytes for guidance as to the 

 origin of foliar development in the sporophyte, for they are the 

 most primitive plants with leafy sporophytes. They may be dis- 

 posed according to the prevalent size of their leaves in a series, lead- 

 ing from microphyllous to megaphyllous types. I have lately shown 

 that such a seriation is not according to one feature only, but that 

 certain other characters which have been summarized as "filicineous" 

 tend to follow with the increasing prominence of the leaf: this 

 indicates that such seriation is a natural arrangement. Now it is 

 possible to hold either that the large-leaved, fern-like plants were 

 the more primitive, and the smaller-leaved derivatives from them 

 by reduction; or conversely, that the smaller-leaved were the more 

 primitive, and the larger-leaved derivatives from them by leaf- 

 enlargement: other alternative opinions are also possible, such as 

 that the leaf-origin has been divergent from some middle type, or 

 that the leaves of vascular plants may have been of polyphyletic 

 origin. 1 For the moment we shall leave these latter alternatives aside. 



1 The view recently advanced by Professor Lignier (Equisetales, et Sphenophyl- 

 lales, Leur origine filicinienne commune, Bull. Soc. Linn, de Normandie, Serie 5, vol. 



