THE FRUIT OF OPUNTIA FULGIDA. 43 



the petals is not very different from that of the inner surface of the petals in 

 many flowers of this cactus. A section through such an abnormal flower-bud 

 gall taken in April shows from a few to dozens or sometimes scores of the 

 small pupge of Asphondylia within the gall. They are embedded in the 

 cortical region of the ovary, chiefly in the portion above the small ovarian 

 cavity, and they stand perpendicular to the surface of the bud. 



In spite of the rather normal appearance of many of the flower-buds which 

 have wintered over unchanged from the preceding season, they never open to 

 flowers. The nearest approach to this process is found in the curling open 

 of the tips of the petals as the buds finally wither (fig. 81). The only open- 

 ing of these galls that does occur is of quite a different sort. During May, 

 especially in the latter half, the pupse of the cactus fly transform to imagos 

 and break out through the epidermis of the bud, each independently. The 

 pupa-cases are left with half their length projecting beyond the surface of 

 the gall (fig. 81). The fly itself perches on the gall while its wings are 

 hardening (figs. 81, 84), and then flits off to play its part in infecting the 

 new flower-buds of the season, which are then just pushing out of the areoles 

 of the vegetative joints. By the end of May a large share of the emptied 

 galls have withered and dropped to the ground, where they decay, often by 

 the aid of fungi, which have ready entrance about the old pupa cases. 



We come now to the consideration of the possible relation between the 

 gall fruits or gall buds and the apparently persistent fruits. As was stated 

 above, many of these latter fruits contain apparently good seeds. Others 

 have only withered rudiments of seeds, and thus resemble certain of the 

 gall fruits. In fact, a complete series of structures can be discovered 

 grading almost imperceptibly, in both external and internal features, from 

 typical fruits to a persistent normal-appearing flower bud. This, together 

 with the fact that the normal type of persistent fruit in 90 per cent of the 

 cases occurs on plants that also bear gall fraits, suggests that both have a 

 common cause (figs. 82, 84). 



If it is the presence of the egg or larva of Asphondylia or of some sub- 

 stance deposited with the egg that inhibits the normal development of the 

 flower, but stimulates it to an abnormal and locally excessive growth and also 

 causes it to persist over winter, then some lesser amount or degi-ee of this 

 same stimulus may be responsible for the persistence of the apparently nor- 

 mal fruits. Such stimulation might occur either by transmission of some 

 substance or of some stimulus from an infected fruit to a neighboring one, 

 which had not been infected. It is also possible that a fruit which had been 

 bored for the deposit of a single egg, or a few, might continue to develop 

 normally in every respect, except that it persisted on the tree. The only way 

 to distinguish between these two possibilities would be by experimental 

 study. The latter of the two \aewt) stated seems supported by the observed 

 fact that the buds containing most pupa? are generally the ones most modified 

 in structure, while buds or fruits with few pupse are relatively little 

 modified. 



