24 THE FKIIIT OF OPUNTIA FITLGIDA. 



during the same season, or more probably for two successive seasons, forming 

 a vegetative axis in tbe first and a flower and fruit in the second. This at 

 least is the most plausible explanation that suggests itself of the origin of 

 such structures as those shown in figures 1c and 88, where nothing but a very 

 slight constriction separates a well-developed fruit from a vegetative joint of 

 the usual length of a year's shoot. 



This same sort of change in the nature of the product of a growing-point, 

 either between the beginning and end of the same growing-season or in suc- 

 cessive seasons, will probably prove to be the explanation of the origin of 

 certain combination joint-fruits found in the flat- jointed opuntias. 



The most interesting problem concerning the development of joint and 

 fruit in these Oacti is, of course, that of the cause determining that, up to a 

 certain stage in the history of the growing-point of each flower-rudiment, 

 there shall be formed photosynthetic leaves, tubercles, and areoles only, while 

 beyond this point the course of development is so changed that thereafter 

 nothing but floral structures are laid down about the margin of this identical 

 group of initials. The fact that this change in the character of the rudi- 

 ments produced on the growing-point occurs at different times in different 

 flowers of the same plant seems to indicate that the conditions controlling 

 this are somewhat local in nature. Such experimental attempts as were 

 made to change the fate of the structures organized about the growing-points 

 of very young flowers, by removing the persistent fruits bearing these young 

 flower-buds, gave no clue to the cause or nature of the change in character of 

 these rudiments. These experiments did, however, show that when the 

 change has once occurred the character and fate of these buds of the areoles 

 are not reversible. In other words, if a fruit of Opuntia fulgida is plucked 

 in February or March and placed in moist sand, certain of its areoles 

 will give rise to vegetative shoots. If, on the contrary, the fruits were picked 

 and planted in April, after the floral structures have been initiated, these 

 same areoles wither without giving rise to any permanent structures (*. e., 

 shoot-buds) such as would have been formed by these very same areoles if 

 picked a few weeks earlier. Further experiments are being undertaken in 

 the hope of discovering the material or responsive basis of this change. 



It would seem, then, that we have strong evidence, from its structure and 

 development, for believing that the present type of fruit in Opuntia has 

 arisen from an originally superior ovary which has progressively sunken 

 more and more into the upper end of the joint bearing it. However, as 

 Harris (1905) has pointed out, there is as yet no adequate evidence for con- 

 cluding with Tourney (1905) that this submergence has occurred very 

 recently in the phylogeny of the genus or family. It is still possible, of 

 bourse, that Opuntia is a less-modified type of a series of which Cereus, 

 EchinocereuSj and Echinocactus are more highly evolved members. In the 

 latter genera the wall of the ovary may bear numerous bract-like leaves 

 resembling those of the opuntias and often also bear axillary areoles, having 

 more or less abundant trichomes and in some cases one or several spines. 



