1958] 



ANATOMY OF GUAYAXA MUTISIEAE PART II 



181 



chaeta) do have them, and since they are relatively similar in anatomy to inner 

 involucral bracts, rather than being bristle-like as in some Mutisieae and 

 Cynareae, they are worthy of consideration. Most authors dealing with evolution 

 in Compositae (e.g., Cronquist 1955) regard the presence of receptacular bracts 

 as primitive. 



An entire receptacular bract of Stenopadus cucullatus is shown in figure 45. 

 Along most of its length, a single vascular bundle is present. Upwardly, however, 

 this forms a number of branches. Although at the base of the bract no fibers 

 jacket the vein, the upper portions of the vascular system are intimately clothed 

 with fibers. A transection of a receptacular bract taken above the point of vein 

 branching is shown in figure 49. In addition to the fibers associated with 

 the veins, sclereids are present in the ground tissue of the bract, and the 

 entire epidermis consists of sclerenchyma. This condition is reminiscent of the 

 anatomy of involucral bracts in 8. cucullatus. Likewise, the anatomy of the 

 receptacular bract of 8. kunhardtii (figs. 46— 18) is similar to the structure of 

 involucral bracts in that species in the relative paucity of sclereids. Except 

 in the basal transection (fig. 48), sclereids are rare in the ground tissue, although 

 an occasional epidermal cell (figs. 46, 47) is sclerosed. 



Receptacular bracts of Stomatochaeta follow much the same pattern, as is 

 shown in the transection of a bract of «S Y . cylindrica, figure 50. The epidermis 

 and one to several hypodermal layers on both surfaces consist of thick-walled 

 sclereids. Only a single bundle could be found along the length of the bract 

 in species of Stomatochaeta examined. 



Alt-hough none of the other genera of Guayana Mutisieae possesses receptacu- 

 lar bracts, the curious gochnatinean genus Wunderlichia does have numerous 

 bristles on the receptacle (like some Cynareae), and the writer was interested in 

 determining if- these had similar structure to receptacular bracts of the two 

 above genera. As the transection in figure 51 shows, this is not the case. A dif- 

 ferentiation into an outer region of sclerenchyma and an inner core of thin- 

 walled parenchyma does exist in the upper portion of the bract, but no vascular 

 bundle is present. This, then, represents a very reduced structure compared to 

 receptacular bracts of Stenopadus and Stomatochaeta. 



The problem of whether such bristle-like bracts on receptacles of Cynareae 

 (a tribe placed close to Mutisieae by most authors) are products of splitting up 

 of receptacular bracts or have some other origin is discussed by Xapp-Zinn 

 (1956). He regards them as subdivided bracts in the subtribe Carlinae. al- 

 though he believes that the remainder of Cynareae is in doubt. True vascular 

 strands have been found in receptacular bracts of Carlinae, although only 

 rudimentary strands, or none at all, have been observed in other Cynareae 

 (Daniel 1890; Nissen 1907). Elsewhere in Compositae, vascularized receptacular 

 bracts have been reported only in Heliantheae (Xapp-Zinn; Carlquist 1957d) 

 and Anthemideae (Xapp-Zinn), no previous studies on these structures in 

 Mutisieae having been made. 



Comparing bracts of both involucre and receptacle in Mutisieae to data of 

 previous work, such as that of Daniel and Xapp-Zinn, who dealt exclusively 

 with other tribes, it is interesting to find that Mutisieae are far more richly 

 provided with sclerenchyma in these structures. Moreover, the sclerenchyma 

 figured by Xapp-Zinn in other tribes appears to be at most thin-walled, and 

 none of the. sclereids with occluded lumina such as possessed by Mutisieae have 

 been reported in other tribes. "Where sclerenchyma occurs in other Compositae, 



