THE FRUIT OF OPUNTIA FULGIDA. 55 



of new plants from fallen fruits is the most important means, next to prolif- 

 eration of fallen joints, of the multiplication and dispersal of this cactus. 



The difference between the product of an areole of an attached fruit and 

 one of a fallen fruit that has rooted seems probably due to a difference in the 

 kind of nutritive (organ-building) material brought to the two areoles under 

 these different conditions. It is possible that the presence of roots on the 

 fallen fruit inhibits the formation of flowers by it. 



Persistence and proliferation of the fruit, though not elsewhere as fre- 

 quent as in Opuntm fulgida, is not unknoA\Ti in other species. In Opuntia 

 versicolor, as also in several flat-jointed opuntias, the frequent persistence of 

 the fruit or even of the unopened flower is the result of the puncture of the 

 flower-bud by the cactus fly, which lays its eggs in it. In other cases, like 0. 

 catacantha, the factors that inhibit ripening and induce persistence are as 

 undetermined as they are in the case of 0. fulgida. 



The fact that Opuntia fulgida and other species have series of fruits show- 

 ing various degrees of sterility, from those with scores of seeds to those that 

 are entirely seedless, can not be taken as conclusive evidence that seed-produc- 

 tion is really on the way to complete extinction in these plants. ISTeither is 

 the corollary that propagation by seeds is being replaced by the proliferation 

 to new plants of fallen fruits as significant as it might at first seem. On the 

 contrary, the stem-like character of the fruits in this genus results in the per- 

 sistence of many sterile ovaries, such as would, in many less fleshy angio- 

 sperms, wither and fall off soon after blooming, instead of maturing into 

 seedless fruits, as they do here. 



