THE FRUIT OF OPUNTIA FULGIDA. 53 



SUMMARY AND CONCLUSIONS. 



The fruits of certain opimtias differ from those of other angiosperms, 

 except those of some Australian " bottle-brush " trees, in not ripening and 

 then either opening or falling from the plant when the seeds are mature. 

 On the contrary, the peculiar fruits of these Cactaceae and Myrtacese remain 

 attached to the plant and actively growing for several or many years. 



The persistent fruit of Opuntia fulgida is still more abnormal in another 

 respect, for it not only remains attached, unripened and steadily growing, 

 season after season, but the seeds are never shed from the fruit. Further- 

 more, the matured fruit itself, or even the ovary of the unopened flower, 

 while still attached to the tree, may give rise to secondary flowers and so to 

 other fruits. Four or five generations of flowers and fruits may thus be 

 formed in a single season. Finally, if a mature fruit falls on moist soil it 

 may develop adventitious roots and shoots and thus initiate a new plant. 



This tree-like opuntia has a tuberculate and spiny cylindrical stem and 

 branches, the fleshy joints of which on separation readily sprout to new 

 plants. 



The early development of the ovary of the flower in Opuntia fulgida 

 closely resembles that of a young vegetative joint, and the structure resulting 

 from this early development, with its minute, evanescent leaves, its tubercles, 

 and axillary areoles, is entirely stem-like in appearance. Only with the 

 initiation of the perianth, stamens, and carpels does the fertile joint become 

 distinctly flower-like in character. The ovarian cavity finally becomes com- 

 pletely buried in the stem-like, basal portion of the ovary, by the more rapid 

 growth of this portion upward and inward about the base of the carpels. 

 The whole outer wall of the ovary and fruit is thus a stem in its morpho- 

 logical origin. This is clearly indicated not only by the more general fea- 

 tures of development noted, but also by the identity in details of development 

 and structure of the tubercles and areoles, and of the photosynthetic and 

 water-storing tissues in the stem and fruit. In its physiological capacity 

 for persistence and for proliferation to flowers and shoots, the wall of the 

 fruit shows again its essential identity with the stem. Finally, the graded 

 series of structures intermediate in character between joint and fruit, serve 

 to further emphasize the likeness of the two. 



The development of the flower of this opuntia indicates that it has evolved 

 from one with an originally superior ovary through the progressive submer- 

 gence of this ovary by the more vigorous growth of parts of the fertile joint 

 that were laid down before the carpels themselves were even initiated. This 

 is probably a relatively primitive type of flower among the Cactacese, from 

 which the type found in Cereus and Echinocadv^ has been derived. 



The perianth, the stamens, and the style are cut off from the top of the 

 ovary, a day or two after the flower withers, by the formation of a highly 

 developed, cup-shaped abscission layer. Any cells of the fruit except those 



