THE FRUIT OF OPUNTIA FTJLGIDA. 13 



group of more darkly staining cells, located in the very axil of the young 

 leaf (figs. 12 at x, 14 at x, 16). With the further growth of the base of 

 the leaf the supporting portion of the stem and the tissue derived from the 

 axillary bud itself together push outward and upward (see Goebel, 1889, p. 

 79) to form the young tubercle or mammilla (figs. 14, 15, 50). This com- 

 bination structure grows more rapidly on the outer side, with the result that 

 the growing-point of the areole comes to lie on the inner face of the tubercle 

 (fig. 14). This shoot apex forms a slightly bulging dome of about half the 

 length of the tubercle and facing directly toward the growing apex of the 

 flower (fig. 14). Later, by the growth of the tissues and organs arising 

 from the adaxial side of the growing-point, the upper end of the tubercle 

 becomes directed more outwardly, often at an angle of 45° with the axis of 

 the flower (figs. 15, 17, 19). The growing-point of the areole from this 

 time onward faces almost directly upward (figs. 12, 17, 20, 24). 



The first rudiments to appear on the growing-point of the new axillary 

 bud are the monosiphonous triehomes, which arise on the margin next the 

 leaf. Following these triehomes there appears a nectary and more triehomes, 

 on the same side, and later another series of triehomes on the opposite or 

 inner margin of the growing-point (figs. 12, 17). 



TRICHOMES OF THE AREOLE. 



The first organs to be developed in the areole, after the growing-point 

 itself, are, as noted above, the monosiphonous triehomes. These are 

 developed in large numbers, scores or hundreds, by the proliferation of 

 many adjoining superficial cells about the growing-point. At first they 

 appear around half the circumference of the growing-point on the side next 

 the subtending leaf (figs. 17, 32, 50). Soon afterward others appear, in 

 smaller numbers, on the side of the growing-point next the main axis. 

 When still later a group of spicules appears on this side of the growing-point, 

 and successive nectaries on the abaxial side, both sorts of structures are sur- 

 rounded, and more or less hidden, by the masses of triehomes developed 

 about them. The youngest triehomes, when 3 or 4 cells long, are bent over 

 the growing-point (figs. 10, 50). Later, when they attain their mature 

 length of 8 or 10 cells, they stand up nearly perpendicularly about the 

 growing-point, though they may (especially in the upper portion) become 

 considerably bent or kinked (figs. 50, 51). The mature trichome consists 

 of a single row of from 6 to 10 or 12 cells. It is about 10 or 12 microns in 

 diameter at the base and three or four times this at the top. The basal cells 

 of the trichome are usually cylindrical, with thin, smooth walls, while the 

 upper 3 or 4 cells are often barrel-shaped and have thickened, spirally 

 marked walls (figs. 51, 52). The terminal cell is often oval, with the 

 smaller end upward. In the older triehomes one or more of the terminal 

 cells may have fallen off, leaving the hair with a square end, commonly the 

 open end of an empty dead cell. 



