THE FRUIT OF OPUNTIA FULGIDA. 31 



dermis between the areoles. In this way considerable portions of the sur- 

 face of a fruit 4 or 5 years old may become covered with cork developed 

 below cracks or bruised spots, and the fruit thus come to have the mottled 

 green and gray color characteristic of maturing vegetative joints (fig. 3). 

 The effect of this cork, aside from affording protection to the water-stored 

 cortical tissues, must be also to cut off the light and air from the photosyn- 

 thetic cells of the portion of the fruit covered by it. AVliile lessening its 

 starch-making capacity, the cork is thus of prime importance to the fruit in 

 maintaining it as a supporting and conducting structure for the flowers and 

 persistent fruits that may arise from it year after year. 



The stiiicture of the areole in the young fruit has already been discussed 

 (p. 12). The chief change in the areole with increased age of the fruit 

 is the increase in size, until it may become 3 or 4 mm. broad and 6 or 8 mm. 

 long in the case of those near the top of the fruit. This increase in size is 

 due chiefly to the continued production of hundreds of trichomes year after 

 year. These appear in rather definite concentric circles around the growing- 

 point. Besides the trichomes, the areole also forms additional nectaries, 

 year after year, till at least a score may successively develop and wither. 

 The bristles of the fertile areole, as has been noted, are not increased in num- 

 ber after a flower is initiated (figs. 14, 17, 50). The sort of development 

 here indicated is the characteristic one for many of the areoles of the upper 

 half of the ovary throughout the life of the fruit. Others of the basal half 

 or quarter of the fruit may persist, but show only slight growth for 2 or 3 

 years and then cease growing altogether. 



Other of the upper areoles, several in each fruit, give rise to one or the 

 other of the two most important structures developed from fimits. These 

 are the floral shoots that may arise on the attached fruit and the vegetative 

 shoot that develops from the areole of a detached fruit. These structures 

 are, of course, the normal products of axillary buds. Ilence the earlier 

 period, during which these areoles are developing only modified structures 

 like bristles and nectaries, must be regarded as a sort of resting-period in 

 which the normal activity of the bud is inhibited. 



When the normal development of an areole is accomplished and a shoot 

 produced, all further activity by this areole is terminated, as must be evident 

 from the fact that the whole mass of the single growing-point of the areole is 

 embodied in either the vegetative shoot or the flower. The exact conditions 

 conducive to the production of a flower in one case and a vegetative shoot in 

 another, with the details of development of each, will be described, as far 

 as knowTi, in the discussion of proliferation (see pp. 35-50). 



The possible activities of the fruit of Opuntia fulgida may be summarized 

 as follows: The fruit usually remains attached and in succeeding seasons 

 buds out new flowers. If fruits fall to the ground from increasing weight 

 of the cluster or when dislodged by wind or browsing animals, then any one 

 of three things may happen. Most frequently the fruit dries up or decays 



