04 Leaf of Vascular Cryptogams and Gymnosperm*. [May 29, 



Finally, in the Cycadacece the rounded apex of the phyllopodium is 

 covered by a definite layer of dermatogen ; its growth is never very 

 distinctly apical, though in Cycas, and perhaps iu Dioon, it is more 

 so than in other genera ; it is winged throughout. Its branching is 

 in all cases Monopodial, but the order of succession is, with exception 

 of the upper pinnae in the above-named genera, basipetal. 



From the above and other facts, it is clear that in the series of 

 plants above dealt with, a progressive differentiation may be traced of 

 the phyllopodium on the one hand as a supporting organ, and on the 

 other hand of the other members of higher order, which develope as 

 flattened organs. The phyllopodium in the first named families is 

 typically a flattened organ with unlimited apical growth ; in the 

 higher members of the series it is a solid structure from the first, and 

 its apical growth is arrested at an early period. Moreover, while in 

 the lower forms it branches dichotomously, in the higher members of 

 the series it branches monopodially, and is thus clearly distinguished 

 at all times from the members of higher order which it bears. It is 

 thought that this progressive differentiation of the phyllopodium as a 

 supporting organ among the similar members of a branch system may 

 throw light upon the mode of origin of the axis as a structure bearing 

 leaves ; and it is suggested that this analogy is at least closer than 

 any that can be drawn from the study of the leafy Muscinece. As the 

 phyllopodium gradually asserts, and in the higher forms of the above 

 series, maintains its identity among the branches of the leaf, so the 

 axis may have differentiated itself as a supporting organ from among 

 members similar to itself in origin and development. 



On ascending through this series of plants, there is further seen a 

 gradual restriction of the apical growth, which in the simpler ferns is 

 sometimes unlimited. This leads on towards the higher vascular 

 plants, in which the apical growth of the leaf usually ceases at an 

 early stage, the general conformation of the leaf being due, as above 

 pointed out, in much greater degree to intercalary growth. Since 

 this change is gradual, no important difference of morphological 

 treatment of the leaf in the lower and the higher forms ought to be 

 based npon it. Now the leaves in the lower forms naturally lend 

 themselves to a treatment throughout as branch systems, while there 

 is even in some complicated fern leaves no structure which can 

 warrant a distinction of the foliar base from the upper leaf. On these 

 grounds, and in the light of this comparative study, it is concluded 

 that the recognition of the phyllopodinm, and treatment of the whole 

 leaf as a simple branch or as a branch system, is in accordance with 

 the true nature of the leaf as seen in all vascular plants : while at the 

 same time, if this method were adopted, the whole shoot would then 

 be subjected to consistent morphological treatment, since the relation 

 of the pinnaa to the phyllopodium is similar to that of the whole leaf 



