THE FEUIT OF OPUNTIA PULGIDA. 27 



tial layers of xylem elezneuts. Outside of these lies a group of 6 or 8 layers 

 of radially arranged cambium-cells and their little modified derivatives. 

 Still further outward on the same radius is the haK-cylindrical strand of 

 phloem-cells, about 15 to 20 cells thick radially (fig. 44a). From a ring 

 of anastomosed bundles near the rim of the funnel at the top of the fruit 

 a series of downward-growing branches gives rise to the 6 or 7 pairs of 

 placental bundles, one pair running longitudinally behind each double row 

 of ovules (figs. 11, 22, 23, 26, 28, 38, 39, 41). From this same vascular 

 ring numerous small branches grow inward and upward. These are the 

 ones that supplied the stamens and perianth members before they were shed. 

 Other similar branches grow still farther downward and then turn upward 

 to form the 6 or 7 bundles of the style and stigmas (figs. 20, 22, 23, 28, 37, 

 41,61). 



The most important structures within the fruit — the seeds — arise, as we 

 have seen, in a double row on each of the 6 or 7 placentae. There are from 8 

 to 12 or more young seeds in each row, making from 100 to sometimes 200 

 or more seeds that are initiated in the fruit as a whole. Usually not all of 

 these mature. Sometimes, as noted above, none of them mature. At the 

 time the perianth is dropped there is a decided difference in size among the 

 seeds present in any one ovary. The most developed are about 0.5 nun. in 

 diameter inside the pocket of the funiculus in which each seed is inclosed 

 and about 0.25 mm. thick. The seed-coats are unthickened and have only 



2 layers of cells each. The embryo sac has usually reached the 8-nucleate 

 stage. The cavity of the ovary is at this time about half filled with the seeds 

 and the swollen funiculi or seed-stalks. 



THE MATURE FRUIT. 



At the end of the growing-season, which in Tucson terminates about the 

 middle of October, the fruit has attained what must be regarded as a mature 

 stage. The seeds within it are now full grown. These mature fruits, of from 



3 to 5 months' growth, differ considerably in shape and size. They are 

 commonly pear-shaped, or somewhat more rounded, and have a diameter of 

 12 to 15 mm., with a length of 20 to 35 mm. or more (figs. 3', 7, 8). Later 

 fruits of the season may be as large as, or even larger than, the earlier ones. 

 Fruits of one season may average in all dimensions larger than those of the 

 season before. 



The surface of the typical fruit at maturity has filled out so that the 

 tubercles have lost their prominence. The areoles differ in size on the same 

 fruit, the upper or larger ones at this time being 15 to 30 or even 35 Trim, in 

 diameter. Occasionally an elongated fruit will be formed which has much 

 the appearance of a mammilla t© vegetative joint, except for the presence of 

 the perianth-scar at the top and the fact that the spines are weaker than in 

 most joints (fig. 7). 



The most important features of the internal structure of the fruit are those 

 concerned with the vascular bundles of the wall of the ovary and with the 



