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 areolae 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 areolae 

 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 

 (Pis. XXXIX, XL, XLI), the entire blade contracts, and the 

 blade's margin recedes sufl&ciently to nearly reach the paracostal 

 areolae 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 areolae outside the paracostal areolae had 

 been cut off the leaf, and the few veins remaining outside the 

 paracostal areolae had thus been rendered mostly free. The 

 veins forming the outer edges of the paracostal areolae become 



