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MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



[VOL. 10 



form the helical wall thickenings (fig. 41, enlarged in fig. 40). These bands, 

 which are homologous with the typical helical or annular bands on protoxylem 

 tracheitis, are up to half the lumen in width. In helical tracheids more than one 

 helix (usually two) may be present per tracheid. The bands are thicker toward 

 the center of the cell, reflecting a "bordered" condition, as seen in figure 40, 

 left. As seen in a transection of the leaf tip (fig. 42), the tracheids appear like 

 fibers, because the wide bands resemble the thick walls of fibers. There seems 

 little doubt that these represent specializations of ordinary annular and helical 

 protoxylem tracheids. The transition to them from normal tracheids is abrupt, 

 but elements with transitional width of thickening bands may in fact be found. 

 The elements with the widest bands occur at the most apical portion of the leaf 

 tip. The function of these peculiar tracheids is difficult to imagine. There is 

 no hydathode formation in the terminal portion of the leaf (which is covered 

 with one to several layers of hypodermal sclereids), nor any stomata in the 

 terminus, so that this function may be ruled out. Although these tracheids are 

 probably rare in angiosperms at large, they have been described in Cactaceae, 

 as the account of that family in Metcalfe and Chalk's Anatomy of the dicoty- 

 ledons shows (Cf. also A. Brongniart, Arch. Mus. Hist. Nat. Paris 1: 405-461. 

 1839). 



Orectanthe. 



Leaves of Orectanthe are basically not unlike those of the species of Abolboda 

 just described, but differ from them in having a broad, flat shape, and an 

 abaxial hypodermis (in some places, at least) as well as an adaxial one. The 

 chlorenchyma cells are elongate in longitudinal section, and have symmetrically 

 placed short arms (fig. 43). They correspond to the cells shown for Xyris lanata 

 by Solereder and Meyer (1929, fig. lib). 



1. 0. ptaritepuiana (fig. 29). In this species, 3-4 cell layers of hypodermis 

 are present on the adaxial surface. The same type of parenchyma cells which 

 comprise the hypodermis are present as a sheath extension on the larger veins, 

 and as a single layer of hypodermis on the abaxial surface between the bundle- 

 sheath extensions (at least in some places). The remaining portions of the 

 mesophyll are pockets, oriented longitudinally in the leaf, which contain a uni- 

 form tissue composed of the peculiar chlorenchyma cells described above. The 

 smaller bundles which do not have sheath extensions are embedded in this tissue. 

 The bundles are elongate in transectional outline, and are completely ensheathed 

 by a sclerified bundle sheath. These bundles are strictly collateral in structure. 



2. O. sceptrum. Both typical O. sceptrum and subsp. occidentalis (fig. 30) 

 are alike in having leaves thinner than those of 0. ptaritepuiana. They are like 

 leaves of 0. ptaritepuiana in all details of anatomy except that (1) they often 

 have an abaxial hypodermis nearly as wide as the adaxial one, (2) they have a 

 prominently sclerified epidermis, and (3) the bundles are round, not elongate, 

 in outline. The thickenings on the walls of the epidermis are narrowest on the 

 outer wall. In upper levels of the leaf of 0. sceptrum subsp. occidentalis, the 

 parenchyma of the hypodermis and bundle-sheath extension was found to be 

 more thick-walled and lignified. A section of the sheath portion of the leaf in 

 this subspecies reveals that the structure of this portion of the leaf is identical 

 with that of the leaves of the larger-stemmed abolbodas. 



