LEAF IN THE VASCULAR CRYPTOGAMS AND GYMNOSPERMS. 
571 
study is fully to justify the treatment of the whole leaf as a branch system, while it 
brings into greater prominence the fact that the main axis of the leaf in the more com¬ 
plicated forms has undergone a progressive differentiation as a supporting organ, as dis¬ 
tinct from the branches of higher order which it bears ; further, that as we pass upwards 
through the scale of vascular plants, there is a tendency to an earlier arrest of the apical 
growth of the phyllopodium ; this, together with the increased prominence of the results 
of intercalary growth, which has in many cases so distorted the branching system of the 
leaf, gives a ready explanation of the origin of those methods of treatment of the leaf, 
which 1 have alluded to above. 
Comparative Study oj the Leaf. 
Taking first that family of the Leptosporangiate Ferns'” in which the simplest structure 
of the vegetative organs is represented, viz., the ILymenopliyllacece, the phyllopodium 
is often found to be obviously winged throughout its length, and continuously to the 
point of insertion on the axis {e.g., Hymenophyllum ciliatum, Trichomanes radicans), 
* The term “ Leptosporangiate ” was first introduced by Goebel (Bot. Zeit., 1881, p. 717) to include 
those Ferns in which the sporangium originates from one cell ; these are further distinguished by the 
whole structure of the sporangium, the regular succession of its cell divisions, the form of the arche- 
sporium, &c., from the Eusporangiate forms, in which the sporangium does not originate from a single 
cell, and is of more complicated structure. He puts forward the following classification:— 
I. Leptosporangiate Forms. 
a. Filices. 
(1) Homosporous Ferns. (Polypodiacese, Gleicheniaceae, Cyatheaceee, &c.) 
(2) Heterosporous Ferns. (Salviniaceae.) 
B. Marsiliacece. 
(1) Marsilia. 
(2) Pilularia. 
II. Eusporangiate Forms. 
A. Filicales. 
(1) Marattiaceae. 
(2) Ophioglossaceae. 
B. Equisetinece. 
(1) Catamites. 
(2) Equisetacese. 
C. Splienopliyllece. 
D. Lycopodince. 
(1) Lycopodiacero. 
(a) Homosporous forms. Genus Lycopodium. 
(b) Heterosporous forms. Lepidodendron. Sigillaria (?) 
(2) Psilotacese. 
(3) Selaginellese. 
(4) Isoetas. 
E. Gymnosperms. 
4 P 2 
