532 Harris: The fruit of Opuntia 



Mammillaria gracilis produces small, nearly spherical, lateral proc- 

 esses of the size of a hazel-nut, which are readily detached, even 

 by a dash of water, or may easily cling to animals- On 0, gracilis, 

 according to him, one rarely finds flowers and still more rarely 

 fruits ; the plant depending apparently entirely upon the brittle 

 branches, which may even be broken off by the wind, for propa- 

 gation. He also mentions <9. curassavica, 0. Biglovii and 0, 

 aurafitiaca as forms in which the easily detached branches readily 

 take root and reproduce the plant. 



More recently Tourney himself has published * upon vegetal 

 distribution in Opuntia and finds that of fourteen species of Cylin- 

 droptmtia examined in the field all are more or less adapted for 

 dissemination in this manner. The fruits of the larger number of 

 the species are not sterile, but " as a generalization it may be 

 stated that with this great group of plants the adaptations for 



1 



vegetal dissemination are inversely as their seed production." 



In a general study of vegetative distribution among plants, Ter- 

 racciano pays particular attention to the Cactaceaey\ confining his 

 observations especially to Opnntiae which he was able to observe 

 under cultivation. He recognizes four types of vegetative repro- 

 duction and dissemination in the genus and adduces several illus- 

 trations of each. 



+ 



As to Tourney's conclusions concerning vegetative dissemi- 

 nation, there can be no question of the importance of vegetative 

 propagation in the Cactaceae, and we may possibly regard some of 

 the structures described as adaptations which have arisen because 

 of their usefulness to the species. At the present time perhaps 

 the tendency is to place too little rather than too much weight 

 upon the idea of adaptation, but it seems that much more evidence 

 should be assembled before the suggestion, that the pendant groups 

 of fruits or the spineless branches resembling them have devel- 

 oped because of the intimate relation of animal life and vegetal 

 dissemination, can be admitted even into ecological theory. 



The conclusions concerning the fruit are as follows : 



1. The fruit of Opuntia is caulome in structure. 



2. Its caulome nature is probably of recent development. 



*Touuiey, J. W. Bot. Gaz. 20 : 356-361. 1895. 



f Terracciano, A. Cont. alia Biol. Veg. 3: 52-59, 67-68. 1892. 



