cook: morphology and evolution of leaves 543 



clearly the two kinds of petioles. Both kinds are represented 

 among the palms. In fan-palms the so-called petiole is below 

 the ligule, whereas the part described as petiole in some of the 

 pinnate-leaved palms certainly is above the ligule or ocrea. In 

 the fan-palms it is plain that the petiole is a narrowed, elongated 

 portion of the leaf sheath, ending at the ligule, whereas the 

 petiole of the Cocaceae and Chamaedoreaceae represents a naked 

 basal portion of the rachis or midrib. 6 



That the petioles are of two kinds is apparent also from the 

 fact that in the fan-palms the petioles agree in structure and are 

 entirely continuous with the leaf sheath, while in the pinnate- 

 leaved palms there is equally complete agreement and continuity 

 with the rachis. But the leaf structure is not the same in all 

 of the pinnate-leaved families. There appear to have been sev- 

 eral independent derivations of pinnate leaves from fan-leaved 

 ancestors. The Geonomaceae are a pinnate-leaved family in 

 which the petiole appears to be a part of the sheath, the same as 

 in fan-palms. 



The magnolia family affords another example of double differ- 

 entiation of the petiole, the lower part of the organ being formed 

 by a thickened segment of the leaf sheath, while the upper part 

 is a narrowed base of the blade, as shown by the decurrent 

 margins which run down to the ligule. In such species as Mag- 

 nolia virginiana the lower portion of the petiole is marked very 

 distinctly by the scar of the deciduous leaf sheath, with a minute 

 hairy prominence at the end, which may be considered as a thick- 

 ened base of the ligular prolongation that forms the apex of the 

 sheathing bud scale. 



It is conceivable that a petiole formed originally from the 

 sheath element might become separated from the remainder of 

 the sheath, which would account for the evolution of such organs 

 as the deciduous bud scales of the Artocarpaceae. But all 

 petioles formed from sheaths would remain essentially different 

 morphologically and developmentally from those that were 

 formed originally as basal elongations of the midrib of a simple 



6 Cook, O. F. Origin and evolution of angiosperms through apospory. Proc. 

 Wash. Acad. Sci., 9: 174. 1907. 



