44 THE FKTJIT OF OPTJNTIA FULGIDA. 



In otter species, such as the flat-jointed Opuntia discata, the disturbance 

 of the normal development of the flower and fruit by the laying of the eggs 

 of Asphondylia is never so marked as in Opuntia versicolor. The perianth 

 of this species is always shed, leaving a definite scar. This probably means 

 that it develops to maturity and then opens more or less completely. The 

 general form and size of the resulting fruit are relatively little affected (fig. 

 8Y). The ripening of the fruit also is only partially inhibited, though the 

 complete ripening and subsequent fall of the fruit are rarely accomplished 

 normally. In one example found at Tucson a fruit of 1913, bearing 50 or 

 more Asphondylia scars, persisted and bore a vegetative joint 5 cm. long 

 in 1914, and both were plump and green when collected in May 1915. 

 Usually the fly escapes from its pupa-case and the riddled fruit withers as it 

 does in Opuntia versicolor. 



Griffiths (1913, p. 18) reports that a similar retention of the gall-like 

 flower-buds or fruits in Opuntia puberula is due to the attack of the black 

 opuntia louse. 



If the explanation suggested above for the retention and modification in 

 structure of the flower-buds of Opuntia versicolor is the real one for this 

 case, then it is quite possible that a similar one may be found for the similar 

 peculiarities of 0. fulgida. That is, it is conceivable that some relatively 

 slight change or lack of change in the nature of the cell-sap, or in the char- 

 acter of photosynthetic or other metabolic products of Opuntia fulgida, from 

 a cause as simple as the sting of a fly, though as yet undiscovered, may prove 

 the adequate explanation of the peculiarities of the fruit of this cactus. 

 Experiments have been initiated to test this hypothesis by the aid of injec- 

 tions and by otherwise changing the external or internal conditions affecting 

 the plant. They have not yet been completed. 



We have still to discuss the marked diversity in behavior, under the same 

 conditions, of the different axillary buds of the flower or matured fruit, and 

 also that of the same buds of the fruit under different conditions. If a 

 mature fruit of Opuntia fulgida is taken from the tree at any time from 

 October to April and placed on damp soil, some of its areoles will push out 

 to vegetative shoots. On the other hand, if the same fruit were to be left on 

 the tree till May no shoots at all would be formed, but the same areoles might 

 give rise to flowers instead of vegetative shoots. Moreover, while only the 

 areoles of the distal quarter of the flower or fruit develop so long as the fruit 

 remains attached, any of the buds, except those of the very basal quarter of 

 the fallen fruit, may give rise to root or shoot. The areoles which actually 

 do this are determined by the position of the fruit on the soil. Finally, if 

 these same fruits are taken from the tree aboiit May 1 and planted, the 

 areoles which had already begun to swell slightly do not go on to develop vege- 

 tative shoots, but may either fail altogether to develop or give rise to small, 

 imperfect flower-buds, which soon wither and drop off. Later, other and 

 more basal areoles may proliferate to vegetative shoots. 



