LEAF IN THE VASCULAR CRYPTOGAMS AND GYMNOSPERMS. 
005 
plants intermediate between the Muscinece and the Filicinece have been found living 
upon the earth, which might give a key to the origin of the morphological differentia¬ 
tion of the sporophore. The series passes at a single plunge from cellular structures, 
with no differentiation of axis and leaf (sporogonia), to vascular plants, with a well 
differentiated axis and leaf: thus we can only speculate by analogy as to the mode of 
origin of the differentiated stem and leaf in vascular plants. The analogy of the 
morphological differentiation of the oophore in the leafy Muscinece is well known, and 
too often applied as though the “ leaf ” in that group were the homologue of that in 
the vascular plants. I think that, in the above comparative study of the differentia¬ 
tion of the phyllopodium from the pinnae which it bears, we have presented to us a 
truer analogy than that of the Muscinece. May we not with good reason think that, 
just as the phyllopodium gradually asserts itself as a supporting organ among 
structures originally of similar origin and structure to itself, so also the stem may 
have gradually acquired its characters by differentiation of itself as a supporting 
organ from other members originally similar to itself in origin and development ? 
Thus the stem and leaf would have originated simultaneously by differentiation of a 
uniform branch-system into members of two categories, and this is what is actually 
illustrated, in the case of the phyllopodium and pinnae, in the series of plants above 
discussed. 
The most prominent change to be seen in the mode of development of the leaf 
on passing from the Ferns to the higher vascular plants is the restriction of that 
apical growth which is so prominent a characteristic of Ferns, and consequently 
the greater prominence of the intercalary growth in those of the higher plants which 
have comparatively large leaves. The observations detailed above show that while 
the phyllopodium of the Leptosporangiate Ferns has a continued apical growth, which 
is sometimes unlimited, that of Angiopteris is, at least in some cases, arrested at a 
comparatively early period : again in Cycas, anc probably in Dioon, the growth at the 
apex and acropetal development of the pinnae are continued for a short time, while in 
most other Cycadacece the apical growth ceases at a still earlier period. 
Thus there are intermediate stejos between the Ferns with continued apical 
growth of the leaf, and the higher plants in most of which that apical growth is 
arrested at an early period. 
In the Ferns the intercalary growth is carried on simultaneously with apical and 
marginal growth, and thus does not play such a prominent part in moulding the form 
of the mature leaf of those plants. But in the higher vascular plants the growth at 
the extreme apex and at the margin is usually arrested at a comparatively early stage, 
and the effect of the intercalary growth more or less strictly localised, which brings about 
various displacements of the original position of members, often becomes more apparent, 
or is even actually greater than in the Ferns. It is chiefly by the variations of external 
form of the leaf, resulting from various distribution of intercalary growth, that the 
leaf of the higher plants acquires its prominent characteristics ; and it is mainly owing 
