542 cook: morphology and evolution of leaves 



between the stipules on the side opposite the petiole is some- 

 times shown in very large stipules of strong shoots. 



Another line of evidence tending to throw doubt on this idea 

 of stipules as specializations of leaf blades might be drawn from 

 bracts and other organs that take the form of sheaths, even in 

 families whose normal foliage leaves are without stipules. Thus, 

 in the genus Hicoria the outer scales of lateral buds have the form 

 of a closed sheath, carinate on each side, somewhat similar to the 

 prophylla of grasses and palms. The large membranous bud 

 scales that precede the leaves on new shoots of Hicoria also 

 represent sheaths and sometimes appear in modified form as 

 stipular wings of the petiole, on abnormal leaves of shoots devel- 

 oped late in the season. 



Considerations drawn from the study of "nodal anatomy" 

 have been used recently by Sinnott and Bailey as the basis of 

 interpretation of leaf morphology. Stipules, sheaths, and ligules 

 are classed together as " modifications of the base of the petiole," 

 the final conclusion being that " a leaf with two distinct stipules 

 is more ancient in type than one with a sheathing base." 4 



Stipules have also been considered as "accessory leaves," or 

 as a specialized development of the basal portion of the "true 

 petiole." 5 In the present view, stipules, bud scales, and various 

 forms of bracts appear to have been derived not from the blade 

 or the petiole of specialized leaves, but from the primitive sheath. 

 The ligule and even the blade itself are considered as specializa- 

 tions from the distal end of the sheath ; but scales and stipules as 

 specializations of the basal portion. Petioles are not all homolo- 

 gous, but are of two kinds, some derived from the sheath, and 

 others from the blade. 



THE TWO KINDS OF PETIOLES 



Taking account of the ligule or the ocrea as marking the end 

 of the primitive sheath makes it possible to distinguish very 



4 Sinnott, E. W., and Bailey, I. W. Investigations on the phylogeny of the 

 angiosperms. 3. Nodal anatomy and the morphology of stipules. Amer. Journ. 

 Bot., 1: 441-453, pi. 44. 1914. 



6 See, Tyler, A. A. The nature and origin of stipules. Ann. N. Y. Acad. 

 Sci., 10: 1-49, pis. 1-111. 1897. 



