52 THE FEUIT OF OPUNTIA FULGIDA. 



vegetative propagating bodies. This raises the question whether the seed- 

 bearing fruit of all opuntias is on the way to extinction, through a progres- 

 sive loss of the seed-bearing function, which may be expected to end in the 

 evolution of a totally sterile fruit, having no propagative functions other 

 than those that can be eqiially performed by vegetative joints. 



It is true that the vegetative joints and both the fertile and sterile fruits 

 resemble each other greatly in their capacity for proliferation. There seems 

 no adequate reason, however, for assuming that either the proliferating habit 

 or the fundamental structure of the fruit is a secondary thing in the evolution 

 of the opuntias (Toumey, 1895). On the contrary, it is natural that the 

 thick-skinned, water-stored joints of these cacti should prove capable of per- 

 sisting on moderately moist soil until rooted deeply enough to secure a water- 

 supply adequate for the starting of a young plant. The fruit being, as we 

 have seen, really a stem in organization, up to the latest phase of its develop- 

 ment, it is also very naturally capable of proliferation to root and shoot. 

 The capacity of joint and fruit for persistence and proliferation is probably 

 as old as the fleshy character of the family. The persistence of the sterile 

 fruits, at least to maturity, is not a really surprising thing, in view of the 

 preponderatingly vegetative and stem-like character of the bulk of the wall 

 of the ovary. Sterile ovaries occur in many species of angiosperms, but in 

 most of these the carpels constitute the bulk of the fruit. Therefore, when 

 the seeds are wanting in these forms, and the carpels as usual fail to develop, 

 no fruit is formed and the flower bud soon withers and drops off. In 

 Opuntia, on the contrary, even if the seeds and carpellary portion of the 

 fruit do fail to develop, the basal stem-like part may go on, practically 

 unhindered in its vegetative growth, and mature quite normally. 



When these facts concerning the comparative structure and behavior of the 

 stem and ovary of Opuntia are considered, in conjunction with the fact that 

 leaves are present on both and with the undoubted similarity of Opuntia to 

 Peireskia, there seems no adequate reason for believing that the fruit of 

 Opuntia is, structurally, an advanced type among the Cactacese. On the 

 contrary, Opuntia and its close relative Peireskia (which also has a leafy 

 ovary, the areoles of which, in the flower, proliferate to secondary flowers) 

 probably show us the simplest type of the inferior (submerged) ovary char- 

 acteristic of the Cactacese. The capacity of the Opuntia fruit for persistence 

 and proliferation is to be regarded as the natural outcome of its original 

 morphological composition — i. e., a joint with an ovary immersed in its apex. 

 It seems clearly not a result of a marked degeneration of a once less stem- 

 like and more constantly fertile fruit, such as has been assumed by some 

 investigators to have been present during the evolution of the genus. The 

 more specialized or highly evolved flowers and fruits, structurally, among the 

 Cactacese are probably to be sought in such genera as Cerent, Rhipsalis, and 

 Mammillaria. 



