THE FRUIT OF OPUNTIA FULGIDA. 23 



other features in which these structures resemble the vegetative joints of the 

 shoot. 



Fifthly and lastly, the whole history of development of the flower and 

 fruit, when compared with that of the vegetative shoot, shows clearly that the 

 whole lower portion of the ovary, up to the perianth, is developed by a convex 

 growing-point, in every respect just as a branch of the stem is. It is evident 

 that if the growing-point of a flower, when once initiated, continues its activ- 

 ity but a short time before the members of the perianth are laid down, the 

 ovary of that flower will have only a short vegetative portion, and the ovarian 

 cavity will occupy a considerable portion of the whole length of the ovary 

 (figs. 23, 25). If, on the contrary, the growing-point that is to form a 

 flower continues longer to form leaves, areoles, tubercles, etc., there may be a 

 long stretch of vegetative axis developed before the perianth and sporophylls 

 are initiated. Thus the ovary proper may occupy but a small portion of the 

 upper end of the whole segTaent or joint which is the product of the continu- 

 ous activity of this individual gromng-point (figs, n, 7c, 28). 



The comparison of figures 17, 19, 22, and 23 shows clearly that the ovary 

 of this cactus is really secondarily submerged by the upward and inward 

 growth of the portions of the axis just below the perianth. The result of 

 this growth is that the ovarian cavity, at first formed on a level with the 

 uppermost areoles (figs. 16, 17), is buried more and more deeply by this 

 gi'owing upward and rolling inward of the parts of the wall of the ovary that 

 were laid down before the cavity of the ovary had appeared. This differ- 

 ential growth, which, during the ontogeny of the organ, buries the originally 

 nearly superficial and terminal ovarian cavity, until it lies near the middle 

 in the older flower and fruit, suggests clearly the probable phylogenetic 

 origin of the type of fruit found in this opuntia. 



A very striking feature of shoot development in these opuntias is the 

 marked constriction usually formed at the limits marking the growths of the 

 same growing-point in successive seasons. This corresponds, of course, to the 

 boundaries marked by the winter bud-scale scars on the shoots of woody 

 dicotyledons. The anatomical reasons for this constriction in these opuntias 

 are not entirely clear. Apparently the protective structures formed about 

 the terminal bud of the annual shoot at the end of the growing-season render 

 the tissue here firm and incapable of swelling out in the following spring to 

 the thickness of the middle portion of this shoot. 



The characteristic origin of the flower in the cylindropuntias is also, as 

 we have seen, from a new axillary bud which develops a flower with its longer 

 or shorter ovary in the same season that it pushes out of the areole. In cer- 

 tain rare instances in Opuntia fulgida, 0. spinosior, and in others seen in 

 0. cylindrlca, growing at Del Monte, California, the part of a flower-forming 

 joint, containing the ovarian cavity, was marked off from the basal portion 

 only by a very slight narrowing of the joint. Such structures seem to indi- 

 cate that the same growing-point may sometimes develop continuously either 



