588 
MR. F. O. BOWER OX THE COMPARATIVE MORPHOLOGY OF THE 
the section, while the cells increase in size, the whole forming the midrib of the pinna. 
Cell-divisions are however continued below the epidermis at the dorsal side of the 
pinna, thus forming the hypoderma (Kraus, Cycadeenfiedern, pp. 321-323); also along 
the ventral surface, and especially at the two angles of the section. Here the cell-divi¬ 
sion is particularly active both in the epidermis and the sub-adjacent tissues, and the 
arrangement of the walls is almost exclusively anticlinal, i.e., in planes perpendicular 
to the external surface. Thus at the margins of the ventral face of the pinna, two 
wings are formed, which consist of a number of internal layers of cells, varying in 
number from 10 near the midrib to about 8 at the margin, and covered externally bv 
the continuous layer of dermatogen. As is well known, a single vascular bundle 
traverses the midrib of each pinna longitudinally, to the apex; no lateral branches are 
given off from it. Accordingly there is no disturbance of the layers of cells in the 
wings by sub-division and formation of procambial bundles ; instead of this the cells 
of the four or five central layers cease their anticlinal division, the cells elongate, and 
those nearest the centre form the transfusion-tissue of Mold. The two layers, 
adjoining the upper and lower layers of epidermis respectively, develop irregularly ; 
cells of the outer layer sometimes forming hypoderma (and this is especially the case 
at the margin) ; sometimes they develop as palisade parenchyma, especially at the 
upper surface. The layer immediately below it accommodates itself to the mode of 
development of the outer layer. The wings extend throughout the mature pinna from 
closely below the acuminate apex to the base, and on the side nearest the base of the 
leaf the wing is continued for a short distance down the pliyllopodium. 
d. Scale-leaves. 
The first leaves which succeed the cotyledons develop only as scale-leaves, and 
leaves of similar form alternate with the later, more fully developed foliage-leaves. 
It has been above stated that in their first origin, and up to the time of appearance 
of the pinnae, there is no perceptible difference between the scale- and foliage-leaves. 
The differences which appear later are as follows :—The pinnae do not advance 
beyond a rudimentary stage, and remain as rounded papiIke ; later the whole of 
the upper part of the leaf with the pinnae becomes dried up (compare Osmunda, &c.). 
The lower sheathing portion of the scale-leaf differs in bulk from that of the foliage- 
leaf, the former being much less massive, while the curve described by the inner sur¬ 
face, as seen in transverse section, is much larger than in the latter (Plate 39, fig. 29, a, b). 
Differences may also be recognised in internal structure, though the plan on which the 
two forms of leaf are constructed is identical; thus the arrangement of the vascular 
bundles is closely similar in scale- and foliage-leaves, but while a band of sclerenchyma 
extends along both the upper and lower surfaces of the basal part of the foliage-leaf, 
that tissue is almost absent from the scale-leaf. 
Thus, as has been pointed out by Goebel (Bot. Zeit., 1880, p. 784), but without 
