Cook : Evolution of Compound Leaves 



81 



Homologies of Stipules and Pinnae 



If the walnuts and hickories had 

 only one pair of lateral pinnae the re- 

 lation of these to stipules might be 

 more apparent. It would hardly be 

 questioned that the base of the com- 

 pound leaf, below the first pinnae, coi- 

 responds to the primitive foot, or 

 sheath element, while the stalk of the 

 terminal leaflet corresponds to the peti- 

 ole of the simple leaves of hickory 

 seedlings, or to the petioles of some 

 of the plants that have simple leaves, 

 as the peach, plum and apple. The 

 correspondence is complete in the wal- 

 nut and hickory bud-scales, where the 

 leaf-base, or foot, takes a broad sheath- 

 ing form, and is still obvious when 

 lateral wings are shown in the inter- 

 mediate forms of leaves, between the 

 bud-scales and the fully developed 

 compound leaves. Only in the large, 

 adult forms of leaves, with the full 

 number of pinnae, are there no traces 

 of stipular wings on the leaf-base. 

 Lubbock has noted the protection of 

 the bud of the walnut by the "dilated 

 base of the leaf-stalk" as a special fea- 

 ture, but the winged bases and stipules 

 of the bud-scales and small leaves ap- 

 parentlv were not observed. (See Fig- 

 ures 9,' 10, and 14.) 



The lowest pinnae of the interme- 

 diate forms of leaves also show special 

 relations with the lateral wings of the 

 petiole. In many cases, especially in 

 the pecan, the wings of the leaf-base 

 are wider above and run out upon the 

 midribs of the pinnae. Moreover, 

 many of the pinnae that have winged 

 midribs are cut away at the base on 

 one side, forming a characteristic broad 

 notch, sometimes strikingly developed. 

 The basal portion of such pinnae, 

 where the lower side is cut away, may 

 also show a strong curvature, or the en- 

 tire lower pinnae may be curved down- 

 ward, so that they appear quite differ- 

 ent from the others. fSee Figure 12.) 



Pinnae Replaced By Stipules 



Pinnae of the intermediate leaf- 

 forms occasionally are replaced by 

 small, transparent, narrowly-pointed 



organs, closely similar to those that ter- 

 minate the lateral wings of the ex- 

 panded bud-scales. No doubt these 

 small organs would have been recog- 

 nized as stipules if they occurred regu- 

 larly in the normal leaves, instead of 

 being developed into pinnae, and the 

 wings suppressed. All degrees and 

 stages of development are found be- 

 tween thin, narrowly-tapering, thor- 

 oughly stipule-like structures and fully 

 developed lower pinnae, of the same 

 size and shape as the others. The 

 extent of foliar development of the 

 basal organs seems especially variable. 

 As shown in Figure 9, some of the 

 stipule-like pinnae are very slender and 

 delicate, and these may consist entirely 

 of pale, semi-transparent tissue, or the 

 lower side may be of thin, stipular tex- 

 ture like the wing of the leaf-base, 

 while the upper side, above the midrib, 

 has a narrow or broad band of green 

 tissue, like that of the normal pinnae. 

 Replacement of pinnae by rudimentary 

 organs is not confined to the lowest 

 pair as shown in Figure 9, or even to 

 the second pair, as shown in Figure 14, 

 but may occur with any of the lateral 

 pinnae as shown in Figures 10 and 11. 



Development of Compound Leaves 



With the lower pinnae thus appear- 

 ing as homologous with stipules, a like 

 origin and development from the prim- 

 itive stipular element is indicated for 

 the other pinnae, especially in cases of 

 replacement of pinnae by small stipule- 

 like organs and by more frequent oc- 

 currence of basal emarginations on 

 other lateral pinnae, or on all of the 

 lateral pinnae, as happens not infre- 

 quently in the pecan. Though no in- 

 dications of lateral wings have been 

 observed on the joints of the rachis. 

 between the successive pairs of lateral 

 pinnae, this is not to be taken as a 

 reason for supposing that the upper 

 pinnae are different from the lower, 

 in view of the complete suppression of 

 wings from the bases of the adult form 

 of leaves. 



Supposing that the other pinnae are 

 of the same nature as the lowest pinnae. 



