Sensitive Fern 151 
both, aside from the production of sori, largely a matter of con- 
traction of the blade and recession of its margin. The difference 
in the venation of the sporophylls of the two is due to the fact 
that in O. sensibilis the venation is more highly developed, since 
midveins are evident in the lobes of the primary segments, 
before transformation of the sterile leaf into a sporophyll begins, 
and that the recession of the margin and the contraction are 
carried further than in L. areolata. 
It seems probable that if this contraction and recession were 
carried in L. areolata, as in O. sensibilis, to the extent of cutting 
the veins that form the outer edges of the paracostal areole, 
shortening the free veinlets thus formed, and so reducing the 
sori, which in L. areolata are borne on these veins, to a minimum, 
the sori would present much the same appearance in L. areolata 
as in O. sensibilis. In the leaves of O. sensibilis intermediate 
between the usual sterile leaves and the sporophylls, in which 
the contraction and recession have obviously been carried less 
far than in the sporophylls, indusia of varying lengths are seen. 
Some of these indusia are prolonged, extend along a vein, and 
are attached to it by the upper margin, as the indusia are in the 
sori of L. areolata; and it is apparently as the indusia shorten 
in these leaves, approaching their form in the sporophylls, that 
the line of attachment shortens and they become detached from 
the vein everywhere but at the lower end, as they are in the 
sporophylls. The indusia in these transitional leaves may be 
vestigial sori, and the prolonged indusia may be evidence of a 
time when the sori had the same form and were attached in 
much the same manner in the ancestors of O. sensibilis as they 
are in L. areolata. 
This species is apparently very old. O. sensibilis fossilis 
