10 THE FRUIT OF OPUNTIA FULGIDA. 



narrows to nothing at its lower end, giving it thus a rather triangular outline 

 in a radial section of the fruit (figs. 4, 17, 28). 



At the time of the opening of the flower these tubercles are from 6 to 10 

 mm. in length, are 4 or 5 mm. wide, and project 2 or 3 mm. at the top. In 

 the younger flower-bud the radial width of the tubercle is greater in propor- 

 tion to its length, longitudinal to the ovary, than is indicated above, while in 

 the mature fruit the projection is much less. In morphological nature, this 

 tubercle of the opuntias, as has been sho\vn by Goebel (1889, p. 79), is the 

 combined product of the growing upward together of the leaf-base and the 

 axillary bud above it. This is clearly indicated by a comparison of different 

 stages in its development (figs. 12, 14, 15, 17). The upper end of the 

 tubercle is somewhat circular in cross-section (figs. 30, 49), and somewhat 

 raised at the margin (figs. 21, 22 ; cf. also, Wetterwald, 1889, fig. 19). At 

 the highest point of the abaxial side of the margin is borne the leaf, while the 

 most depressed central portion of the end of the tubercle is occupied by the 

 flattish growing-point of the axillary bud, which is surrounded by the rudi- 

 ments of trichomes, spicules, and nectaries developed from it (figs, dl), 12, 

 15, 32, 50). It is difficult to see how all the structures above a, at the left of 

 the growing-point in figure 50, can be regarded as parts of a single leaf, as 

 they apparently would have to be if the view of the morphology of the 

 tubercle held by Darbishire (1904, p. 395) were accepted. 



The growing-point of the flower, like that of the vegetative shoot, may be 

 slightly convex in form during the period of the initiation of the wall of the 

 ovary with its tubercles and the leaves borne by them. It may even retain 

 some of this convexity during the initiation of the 16 or more sepals and 

 petals (figs. 12, 13, 14), With the beginning of formation of the stamens 

 and carpels, however, the same growing-point becomes depressed to form a 

 cup narro^^ing in at the upper margin, about which the numerous (250) 

 stamens are initiated. Later still, the margin closes in to form the carpels, 

 which unite above to f oi-m the roof of the ovary, and finally stretch upward to 

 form the style and its 6 stigmas (figs. 15, 16, 17). 



STRUCTURE AND FATE OF THE LEAVES OF THE OVARIAN WALL. 



The leaves of the wall of the ovary are, as noted, about 15 to 40 in number. 

 Each leaf is approximately conical in form, has a slightly flattened base, and 

 is curved inward above to end in a sharply pointed tip (figs. 4, 5, 11, 19, 23, 

 47). The mature leaf of the ovary is only 3 or 4 imn. long and but 1 or 1.5 

 mm. in diameter. It is abruptly constricted at the base to a stalk, which is 

 nearer the ventral side and is barely a third the diameter of the part of the 

 leaf just above (figs, 19, 50). The leaves of the lower third of the ovary 

 do not attain more than half the size mentioned, while those of the upper 

 quarter, which are more or less appressed against the sepals, may be some- 

 what longer and are commonly very much broadened (figs. 23, 47, at right). 

 Before the ovary has reached half the size it attains at the opening of the 



