1958] 



ANATOMY OF GUAYAXA MUTISIEAE- — PART II 



175 



Differences between base and apex are usually in the nature of a greater promi- 

 nence of parenchyma and more numerous bundles near the base. A section mid- 

 way along the length of a bract gives a reasonable idea of the nature of its struc- 

 ture, and so such sections were employed in this study,, unless otherwise noted. 

 The variation from outer to inner bracts, however, may take various forms and 

 must be considered when comparisons among taxa are made. Receptacular bracts, 

 if present, represent the most extreme expression of the series from outer 

 to inner. 



In the genus Stenopadus, variations in involucral bract structure occur 

 chiefly in the relative amount and distribution of sclerenchyma. Figures 30 and 

 31 show diagrammatic transections of outer involucral bracts of *9. council ii and 

 8. cucullatus respectively, and portions of inner bracts are shown in cellular 

 detail in figure 35 for S. campestris and in figure 38 for 8. cucullatus. 



Comparison of outer and inner bracts for a single species, 8. cucullatus (figs. 

 31. 38), shows differences in thickness of sclerenchyma on each face of the bract 

 and in number of bundles present. In both, however, the epidermis and several 

 underlying layers on both sides of the bract consist of thiek-walled sclereids. In 

 the vascular bundles, fibers or isodiametric sclereids surround most of the bun- 

 dle ; they do not, however, separate xylem from phloem. There are few fiber 

 groups in the mesophyll of bracts which are not directly related to bundles. 



In 8. connellii (fig. 30) the distribution of sclerenchyma is quite different. 

 One or two layers on the inner face of the bract are unsclerified, although a 

 thick band of sclereids is present below the surface on this face of the bract. The 

 outer face of the bract is not sclerified, although individual sclereids or nests of 

 sclereids occur throughout the mesophyll of the bract, unlike the condition in 

 8. cucullatus. Sclerification is conspicuous in association with the bundles: the 

 xylem pole of each strand is often completely surrounded, with the phloem lying 

 on the abaxial face of this fibro-sclereid sheath. In addition, a fibrous bundle 

 cap is formed adjacent to the phloem. Inner bracts of 8. connellii, however, are 

 like those of 8. cucullatus in that both surfaces of the bract are sclerenchyma- 

 tous, and although fibers occur in association with the bundles, no sclerenchyma 

 other than this is found in the mesophyll, which consists of thin-walled par- 

 enchyma. 



Stenopadus campestris (fig. 35) does not agree with either of the two above 

 species in that although the outer bracts are like those of 8. connellii, the struc- 

 ture of the inner bracts is different from that of 8. connellii. Sclerenchyma, 

 rather than being present on both faces, is more frequent toward the inner face, 

 and occurs in the form of nests or individual sclereids. Sclereids are much less 

 frequent toward the outer face of the bract. Among other species of Stenopadus 

 investigated, kunhardtii and 8. stipitatus agree with the types of bracts which 

 occur in 8. campestris ; S. obconicus has inner bracts like those of 8. cucullatus. 



A difference was noted above between the excessive fiber development in 8. 

 connellii and the fibrous sheaths of bundles in 8. cucullatus outer bracts, in 

 which an area of fibers (horseshoe-shaped in transection) often surrounds the 

 bundle, but does not separate xylem from phloem. Among the other species of 

 Stenopadus studied, £. obconicus resembles £. cucullatus in this respect, whereas 

 8. campestris, S. kunhardtii, and S. stipitatus are like S. connellii in such char- 

 acteristics of bundles in outer bracts. Bundles of inner bracts tend to be alike 

 in all the species, with a more or less complete cylindrical sheath of fibers sur- 



