THE FRUIT OF OPUNTIA FULGIDA. 41 



Finally, attempts were made to determine by experiment whether an 

 areole that had once borne a fruit could, after careful removal of this, give 

 rise to a vegetative shoot. When from such fmits all the flowers and sec- 

 ondary fruits Avere removed, and all other areoles cauterized, and the fruits 

 then planted in soil, no shoots at all arose from any of their areoles. This, 

 of course, is rather to be expected, with but a single growing-point in each 

 areole. But the latter condition had not been demonstrated histologically 

 at the time the experiment was initiated. Then, too, it could not be assumed 

 impossible for some of the younger, protected cells from about the margin 

 of an areole of the primary fruit that had already produced a secondary 

 fruit to give rise to a new-shoot growing-point, just as cells outside the areole 

 may initiate a new-root growing-point. In a check experiment 21 fruits 

 that had borne flowers were planted after the removal of these and no areoles 

 cauterized. In 10 months one fruit was dead, 21 were living and rooted, and 

 17 of these bore shoots 3 to 4 cm. high. This demonstrated that no wound 

 injurious to the fruit as a whole is caused by the removal of the secondary 

 flower or fruit. On the other hand, the fact that many of the cauterized 

 fruits mentioned took root and remained green and plump for two years 

 demonstrates that cauterization does not seriously injure the fruit as a whole. 



CAUSES AND SIGNIFICANCE OF PERENNATION AND OF THE 

 DIVERSE TYPES OF PROLIFERATION. 



We have described above the unusual persistence and secondary growth of 

 the fruits of this opuntia and the entirely unique power of the proliferation 

 to flowers and vegetative shoots shown by the areoles of its ovary. Beyond 

 these facts of the structure and habit of fruit and areole lie the physiological 

 problems of the causes and significance in the life-history of these striking 

 peculiarities in this species of Opuntia. 



The primary question to be answered is: Why does the fruit of this 

 cactus never ripen like those of most cacti and of the vast majority of other 

 angiosperms ? ITo appreciable light on this question has been obtained from 

 a study of this opuntia itself. Not a single fruit of this species has been 

 seen, not even an abnormal one, that showed any indication of undergoing 

 those changes which characterize the process of ripening in most other cacti. 



The persistence, year after year, of fruits containing mature ripe seeds in 

 attachment to the parent plant is another peculiarity entirely unexplained 

 by observation thus far made on this opuntia. Dropping off or springing 

 open is in the case of most plants a phenomenon closely associated with the 

 process of ripening. The most noteworthy exception to this general rule is 

 that of Callistemon and allied Myrtaceje investigated by Ewart (1907), 

 where the fruits persist almost indefinitely until killed by the cutting off of 

 the water-supply by fire or drought (fig. 80). 



One possible explanation of the persistent greenness and attachment of 

 the fruit is suggested by the study of an interesting abnormal phenomenon 



