22 THE FRUIT OF OPLTNTIA FTJLGIDA. 



surface of the ovary at the base may be much more stem-like tban. at its upper 

 end, having more prominent tubercles and furnished with areolae that not 

 infrequently bear a spine or two each. Not only do the real functional 

 flowers and fruits differ considerably in size and in the relative development 

 of parts, but the semblance to the stem may go so far that the perianth fails 

 to open, or even fails to develop completely, or (in extreme cases) the carpels 

 or ovules or even the ovarian cavity itself may never be initiated at all. 



Perhaps the small, smooth jointlets or pseudo-fruits mentioned by 

 Toumey (1905, p. 531) are to be regarded as the final members of this series 

 of simplified fruits. These pseudo-fruits occur in more or less dense, fruit- 

 like clusters, which Toumey says are particularly abundant in adverse sea- 

 sons, when true fruits are less abundant. Such structures resemble true 

 fruits in the lack of spines and in the less marked tubercles, but differ in 

 having no trace of a flower or a perianth scar at the terminal end. 



These joints (intermediate in character between flowers or fruits and 

 normal vegetative joints) occur still more frequently in the allied species 

 Opuntia leptocauliSj in which practically all degrees of reduction of the 

 floral parts can readily be found. Perhaps as complete a series could be 

 found for 0. fulgida after long search, but the intermediate types are far 

 more abundant in 0. leptocaulis (figs.. 89, 96). In the platopuntias also 

 joint-fruits quite similar in character to those of 0. fulgida are not really 

 infrequent where large numbers of these plants can be examined in the field. 

 The stem-like character of these joint-fruits is still more clearly indicated in 

 these forms, because the normal joints are flat, while the fruits are usually 

 barrel-shaped or obconcial. A number of examples of these unusual fruits 

 of the flat opuntias have been mentioned in the literature, chiefly on plants 

 growing in greenhouses. Most of these abnormal structures have rather 

 typical perianths, sporophylls, ovarian cavities, etc., but the wall of the 

 ovary, instead of being as usual a radially symmetrical structure, grows out 

 on two sides to wing-like expansions. This gives the whole structure the 

 appearance of a disk-like vegetative joint with a thicker ovary embedded in 

 its upper margin. This ovary may be at the very top or down more or less 

 on the lateral margin of the disk. 



Secondly, the internal structure also of the ovary of Opuntia fulgida is 

 essentially like that of the vegetative joint. Thus the organization of the 

 areole, the photosynthetic system, and the vascular-bundle system are alike 

 in the two structures, except for the additional vascular branches supplying 

 the perianth and sporophylls of the flower (fig. 28). 



A third feature in which the fruit of Opuntia fulgida resembles the vege- 

 tative shoots of the same plant is in its ability to persist for some years on 

 the parent plant and to continue to grow in thickness year after year by a 

 well-defined cambium layer. This point is dealt with in more detail else- 

 where (see p. 29). 



Fourthly, the capacity mentioned elsewhere for self-propagation by the 

 attached flower and that of shoot-production by the fallen fruit are stiU 



