LEAF IN THE VASCULAR CRYPTOGAMS AND GYMNOSPERMS. 
G07 
characters resulting from distention, &c., but the variation from the winged 
structure is less marked ; still in many cases the distinction is apparently accentuated 
by the abortion of the upper parts of the leaf, as in the scale-leaves, in which a 
correspondingly greater lateral extension is found. But this apparent accentuation 
of the difference between the hypopodium and the upper part should not cloud our 
morphological vision; the real nature and origin of the hypopodium remain the same, 
whatever variety there may be in the details of its development in the individual leaf. 
The distinction between the mesopodium and epipodium is never very clearly marked 
in the plants under consideration. Thus, though in these lower forms the differentiation 
of the parts of the phyllopodium is not so complete as in many Angiosperms, there 
may still be traced an increasing clearness of their differentiation on passing from the 
simplest forms upwards. 
A similar comparative study of the series of large-leaved Vascular Cryptogams and 
Gymnosperms shows that progressive changes may be noted in the order- and mode of 
origin of the pinnae on the phyllopodium. In the lower forms of the series the order 
of their appearauce is strictly acropetal , whether their origin be by dichotomy, as in 
the Hymenophyllacece, &c., or by monopodial branching, as in Osmunda, Angiopteris, 
&c. This acropetal order of appearance may be traced as feebly represented in Cycas 
and Dioon among the Cycadacece; but even in these forms there is also a simultaneous 
basipetal order of development of the lower pinnae which is more prevalent in most of 
the genera of the Encephalartece , to the exclusion of the acropetal order of succession : 
thus in Mcicrozamia, Ceratozamia, Zamia, and Encephalartos the order of succession 
of appearance of the pinnae is exclusively basipetal, and since the phyllopodium ceases its 
apical growth after the appearance of the first pinnae, subsequent elongation is due to 
intercalary growth. Such a mode of development is not without its parallel among 
the Angiosperms. Goebel cites, as examples of a basipetal order of development of 
pinnae, the leaves of Myriophyllum, Ceratophyllum, Rosa canina, Potentilla reptans and 
Anserinci, Spircea lobata, &c. ; and he further points out that the order of succession 
is not even constant in one and the same genus, as is seen on comparing Spircea 
Lindleyana, with Spircea lobata (‘ Vergl. Entw./ p. 227). These irregularities in order 
of succession, accompanied by intercalary growth of the phyllopodium in length, may 
further be compared with those examples of similar development cited as occurring 
among the Phceophycece (Goebel, l.c., p. 186). It may be noted that the arrest of the 
apical growth of the phyllopodium, and the tendency to develop the pinnae in a 
basipetal succession, progress simultaneously in our series of large-leaved Vascular 
Cryptogams and Gymnosperms, and it can hardly be doubted that the two phenomena 
are mutually connected. 
In most of the Leptosporangiate Ferns the pinnae arise as more or less flattened 
structures, derived from the marginal series of cells, while some of the neighbouring 
cells also take part in the process : in other words they arise along the lines where the 
wings of the phyllopodium are, or will ultimately be. Though, as was shown by Kny, 
