16 THE FRUIT OF OPUNTIA FULGIDA. 



often drop off with the maturing of the fruit, apparently in consequence of 

 the withering of the base of the spine. The number of spines in the spine- 

 bearing areoles of the joint-hke fruits evidently increases, from year to year, 

 much as in the areoles of the stem. 



Each spine consists of a slender, barbed axis or core and a glistening 

 white, striated sheath. The surface cells of the core, in the upper half of its 

 length, project outward and downward at the tip to form the extremely sharp 

 retrorse barbs (figs. 53, 54). The sheath, which at first covers the whole 

 core of the spine with a tightly fitting jacket, is made up of several layers of 

 greatly elongated and very thick-walled cells (figs. 54, 55). As the spine 

 matures it shrinks in diameter and separates from the sheath. The latter at 

 the same time contracts longitudinally, so that its tip is punctured by the 

 point of the spine. Later the basal portion of the sheath splits to several 

 strips, which are soon folded back on themselves in loops (fig. 53). The 

 result of this is that the tips, even of spines developed in the greenhouse, are 

 left naked for several millimeters. 



BRISTLES OR SPICULES OF THE AREOLE: THEIR NUMBER. STRUCTURE, AND FATE. 



On the inner or abaxial margin of each areole there is a crescentic group 

 of barbed, yellow, weak-based bristles, which form the only armament of 

 most fruits of this species. This curved cluster of spicules reaches about 

 one-third way around the growing-point of the areole (figs. 32, 50). The 

 tips of the older bristles lie close against the surface of the ovary just above 

 the areole. Each individual bristle is practically straight, about 50 or 60 

 microns in diameter and 1 to 1.5 mm. long. The surface of the bristle is 

 made up of thick, yellow-walled cells 5 to 8 microns broad by 100 microns 

 long. The outer ends of these cells project slightly outward and sharply 

 downward to form the characteristic barbs which make these bristles such a 

 persistent and irritating reminder of an encounter with the fruits or joints 

 of this cactus (figs. 57, 58). The cells of the interior of the bristles are of 

 slightly smaller diameter, more elongated, with clear and much thinner walls 

 (figs. 58, 59). 



The bristles are the last of the several types of organs to appear in the 

 areole. Even the first of them do not appear till scores of trichomes and 

 one or two nectaries have been developed (figs. 12, 50). The rudiment of 

 the bristle is not, like that of the trichome, of a single row of cells, but is 5 or 

 6 cells across when it first pushes out from the growing-point (fig. 56). 

 The number of bristles in an areole increases with age. At the time the leaf 

 is shed the crescentic cluster about the growing-point may consist of 6 or 8 

 concentric rows of 20 to 30 bristles in each row (figs. 32, 50). In the 

 sterile areole of a four-year-old fruit this number may be double or triple 

 that just mentioned. When once formed the bristles evidently persist indefi- 

 nitely unless dislodged by browsing animals or by the development of a 

 flower or shoot from the areole. The absorption of a growing-point in the 

 production of such a flower or shoot of course puts a stop to the appearance of 

 further bristles from that areole. 



