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 areolae, 

 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 



