THE FRUIT OP OPUNTIA PFLGIDA. 17 



PERIANTH. STAMENS, STYLE, AND STIGMA: THEIR DEVELOPMENT 



AND ABSCISSION. 



The order of initiation of the organs of the flower is an acropetal one. 

 The series begins with the formation of the peg-like leaves, followed by that 

 of the areoles, the sepals, and petals, all from the characteristic convex 

 growing-point like that of the stem. Then with a change in the growing- 

 point to a concave, cup-like shape, the series of floral parts is completed with 

 the initiation of the stamens and carpels, or perhaps we should say vnth that 

 of the placentas and ovules, which are formed deep in the bottom of the cup 

 {cf. also Goebel, 1886). 



The perianth of Opuntia fulgida consists of about 8 sepals, light green in 

 color, and of a like number of petals, rose-pink in color. These sepals and 

 petals are initiated abotit the growing-point in the same way that the leaves 

 of the ovary are, but differ from the latter in the important particular that no 

 axillary buds are developed at the bases of the perianth members and that 

 there is no tubercle formed at the base of either sepal or petal (figs. 17, 22, 

 47). In mature structure also the perianth divisions difl^er strildngly from 

 mature leaves. Even the sepals are considerably broader and flatter than 

 the leaves, with more vascular bundles, while the obovate petals are very- 

 broad and have a far more complex, reticulate vascular system than the 

 leaves (figs. 47, 64). The mature perianth opens after midday (in mid- 

 afternoon according to Lloyd, 1907). It forms a saucer-shaped flower an 

 inch or more across. A few days after opening the whole perianth falls 

 off, set free by the formation of a well-defined abscission layer. 



The 250 stamens of the flower have filaments about 2 or 3 times as long 

 as the anthers. Each stamen arises as a dome-like elevation, 6 or 7 cells in 

 diameter, on the margin of the now concave growing-point (figs. 16, 18). 

 As the stamens develop they bend inward over the growing-point, the 

 youngest ones standing nearly at right angles to the axis of the flower (figs. 

 17, 20). Later they swell at the end, as the microsporangia appear, and 

 gradually become more erect, but not completely so until the flower is open 

 (figs. 22, 23, 61). 



The internal development of the microsporangia is apparently not essen- 

 tially different from that of the typical angiosperm. The upper,, older 

 stamens open first, as they dry out first. The pollen-grains are irregularly 

 globular, with a yellowish, pitted exine of about 4 or 5 microns in thickness, 

 and of a columnar or palisade-like structure when seen in optical section. 



The carpels, the last structures of the flower to be formed, are 6 or some- 

 times 7 in number. This is clearly indicated by the number of rudiments of 

 carpels initiated around the growing-point, by the number of lobes of the 

 stigma, and by the number of placentas in the mature ovary (figs. 17, 31, 

 32, 33, 34). ISTo case was observed with 5 carpels, the number found by 

 Engelmann (1887) in the plants studied by him. The first rudiments of the 

 carpels become evident after about 6 or 7 tiers of stamens have been devel- 

 2 



