60 THE FRUIT OF OPUNTIA FULGIDA. 



Fig. 50. Radial or sagittal section of an areole of a primary flower, showing the 

 growing-point or stem apex of the areole, its bordering groups of bristles 

 and trichomes, two nectaries, and the subtending leaf. X 18. 



Fig. 51. Three typical trichomes from an areole, showing their swollen tops and 

 slender bases. X 45. 



Fig. 52. A single trichome from an areole, showing thick-walled pitted cells of tip. 

 X 110. 



Fig. 53. A mature spine, showing the swelling at base, the barbed tip, and the trans- 

 parent sheath attached for its lower sixth, while the upper five-sixths of 

 it is loose and has slipped down by splitting and folding near the base 

 to leave the tip of the spine exposed. X 5. 



Fig. 54. The upper fourth of an immature spine, showing in detail the intact striated 

 sheath and the barbed tip of the spine itself. X 50. 



Fig. 55. Transverse section of a spine from a young fruit, showing the sheath of 

 loosely packed hairs and the spine with its core of closely compacted 

 small cells and its outer layers of large thick-walled barb-cells. X 110. 



Fig. 56. Part of a sagittal section of an areole, showing a nectary with bristles and 

 trichomes at its base, with the separated, cutinized layer of the epi- 

 dermal cells at its tip; also the vascular supply of the nectary and of 

 the subtending leaf. The leaf is just being cut off to leave a relatively 

 small leaf-scar. X 45. 



Fig. 57. Surface view of upper half of a mature spicule or glochidium, showing the 

 barbs closely resembling those of spines. X 72. 



Fig. 58. Longitudinal section of basal fourth of a nearly mature bristle, showing 

 internal structure, barbs, and zone of rupture at which bristle breaks 

 off. X 125. 



Fig. 59. Transverse section near middle of an immature spicule, showing the large, 

 thickened surface cells which are to give rise to the barbs. X 225. 



Fig. 60. Longitudinal section of upper half of opening flower, showing the most fre- 

 quent location of the abscission layer, which cuts off the whole perianth 

 from ovary, but leaves style to be cut off independently. X 3. 



Fig. 61. Section similar to that in figure 60, showing another type of abscission layer, 

 which cuts off the style as well as the perianth. Separation has already 

 occurred in the left half, while the abscission cells on the right are in 

 the stage shown in figure 67. X 3. 



Fig. 62. Part of longitudinal section of wall of an ovary from which the perianth has 

 fallen, showing surface left after the break through the youngest cells 

 of the abscission layer. The arrow is parallel to the basal part of the 

 style. (Cf. figure 67.) X 48. 



Fig. 63. Part of radial section of perianth-scar of very young fruit, showing phellogen 

 layer and cork. Abscission surface at right. X 117. 



Fig. 64. Part of radial section of perianth scar of a two-year old fruit, showing the 

 parenchyma of fruit at left, phellogen in the middle, and cork at right. 

 X 110. 



Fig. 65. Small portion of section similar to that in figure 64, showing structure of two 

 of the thickened cells in the cork. X 225. 



Fig. 66. Outline of vascular system of inner petal. X 5. 



Plate 7. 



Fig. 67. Part of the radial section of ovary of newly opened flower illustrated in 

 figure 59, showing various stages in the division of cells that are forming 

 the abscission layer. The arrow is parallel to the style. X 110. 



Fig. 68. Part of radial section through ovary of an opening flower, showing the rela- 

 tion of the abscission layer to the vascular bundles and to the mucilage 

 cells. X 19. 



Fig. 69. Part of transverse section of two-year vegetative joint, taken through a 

 tubercle just below its areole, showing the structure of the epidermal 

 and of the palisade-like chlorophyll-containing tissues of the stem. 

 X45. 



