Narrow-Leaved Chain-Fern 141 
stretched as the leaf becomes large, sometimes, the lower espe- 
cially, to the vanishing-point. 
Midveins are rarely evident in the lobes sometimes 
present on the blade’s primary segments, but when evident 
are formed in the same manner and start from such veins as 
in O. sensibilis. 
In the early leaves, the petiole, or at least its upper part, is 
narrowly more or less winged laterally. Later the wings con- 
tract, becoming mere ridges. The contraction apparently begins 
below, extends upward, and involves the basal part of the blade; 
either before or soon after the formation of one or both of the 
two basal primary lobes (whose midveins, if evident, would start 
from the starting-points of the two basal paracostal areole next 
the blade’s primary midvein). These lobes thus apparently 
are sometimes never formed and sometimes aborted early. The 
parts of the blade that contain the two basal paracostal areolz 
next the primary midvein appear to shrink into extensions of 
the petiole’s lateral ridges. The contraction finally involves 
the wings between the blade’s lower primary segments, these 
wings also, in the sporophyll especially, becoming more or 
less ridges. 
In the transformation of the sterile leaf into a sporophyll 
(Pls. XXXIX, XL, XLI), the entire blade contracts, and the 
blade’s margin recedes sufficiently to nearly reach the paracostal 
areole but not, as in O. sensibilis, sufficiently to cut them. The 
effect produced is the same as if the part of the leaf-blade con- 
taining most of the areole outside the paracostal areole had 
been cut off the leaf, and the few veins remaining outside the 
paracostal areole had thus been rendered mostly free. The 
veins forming the outer edges of the paracostal areola become 
