LEAF IN THE VASCULAR CRYPTOGAMS AND GYMNOSPERMS 
597 
similar to the pinnae. The pinnae elongate, but remain straight (Plate 40, fig. 45) ; at 
the margins of the ventral surface a winged development begins, which is continued 
here over the extreme apex of the pinna, thus producing that appearance of a gradual 
fading away of the midrib, towards the apex of the pinnae. 
Turning now to the details of development of the pinna, it is seen in transverse 
sections of pinnae of various ages that at all times the external surface is covered by a 
continuous layer of dermatogen, in which periclinal cell-divisions never occur. At the 
very first appearance of the wings, both periclinal and anticlinal divisions are found 
in the cells lying below the dermatogen ; but soon the former cease, and as the result 
of continued and successive anticlinal division, the wings as they develop are com¬ 
posed as in Cycas of a number of clearly-defined layers of cells, enclosed by the 
dermatogen : their number varies from about eight or more (exclusive of the derma¬ 
togen) at points close to the midrib, to six or five towards the margin. The 
regularity of these layers is subsequently disturbed by the occurrence of periclinal 
walls also, along the course of the future vascular bundles, thus resulting in the 
formation of procambial strands. About the same time as the procambial strands 
make their appearance the margins of the wings become gently serrated, especially 
near the apex : the serration is already clearly marked before lignification begins in 
the wings, but its appearance is subsequent to that of the lignification in the midrib. 
The chief increase in surface of the pinna is by intercalary, longitudinal and transverse 
growth. It is thus apparent that the development of the wings is closely similar to that 
in Cycas, or in Gnetum (infra) ; also that though the ultimate external appearance of 
the pinna resembles that of Angiopteris more than either of the above genera, still 
there is between the two this difference in their mode of development: that whereas 
there is a clearly marked and continuous dermatogen in Stangeria, in Angiopteris there 
are repeated and frequent periclinal divisions in the peripheral cells, and especially in 
those near the margin of the young wing. 
Gnetace^e. 
Gnetum Gnemon. 
Having already prepared notes on the development of the leaf in the Gnetacece, it 
appeared to me better to embody the results in the present paper along with those 
above detailed than to defer their publication ; moreover this course is justified by 
the bearing which those results have upon the general subject now in hand. 
The cotyledons of Gnetum Gnemon develop in a similar way to the ordinary foliage 
leaves. These arise in decussating pairs, and at first appear as rounded papillae : the 
apices of these papillae soon become slightly elongated, so that the form of the whole 
young leaf is acutely conical, while below it is massive (Plate 40, figs. 47-50). At 
first cell-division and concomitant growth go on almost uniformly throughout the 
young leaf, but even at such an early stage as that represented in Plate 40, fig. 47, 
