THE FRUIT OF OPUNTIA FULGIDA. 11 



flower, the lowermost of the 15 to 40 leaves of the ovary have dropped off, 

 many of them withering while mere rudiments, half -grown or less (fig. 20, 

 lower areole). The separation occurs at the constriction mentioned, and by 

 the time the flower is well opened all its leaves outside the calyx have fallen. 

 This separation is apparently not determined by a definite abscission layer. 

 The dropping of the leaf is followed by the shrinking together of the short 

 stump of the leaf-stalk and later by the formation of a protecting scar-tissue 

 of 15 to 20 layers of corky cells (fig. 14). Evidently these leaves of the 

 wall of the ovary, being relatively few, small, and transient, are able, like the 

 similar leaves of the vegetative joint, to play only a very subordinate part in 

 the photosynthetic work of the plant. They are certainly much less impor- 

 tant in this work than the abundant and permanent photosynthetic tissue of 

 the wall itself. 



In correspondence with this relatively unimportant photosvnthetic work 

 of the leaf, its internal structure shows little of the characteristic specializa- 

 tion of an efficient starch-making organ (figs. 45, 46). Stomata are few 

 and scattered, on the under side of the leaf only. Instead of the character- 

 istic palisade found in the leaves of most other plants and in the joints and 

 fruits of Opuntia fulgida itself, we find the whole outer region, especially on 

 the dorsal side of this leaf, made up of nearly isodiametric cells, among 

 which are scattered small cells containing calcium oxalate crystals and much 

 larger cells filled with slime or mucilage. The latter are of the type that 

 will be described in more detail when we come to the consideration of the 

 internal structure of the fruit (figs. 45, 46). ISTear the fiattened base of the 

 leaf (fig. 45), five vascular bundles are to be seen in its cross-section, but 

 only the middle one of these reaches to the tip of the leaf. The subordinate, 

 lateral bundles are made up chiefly of short, broad, thick-wialled elements 

 which are often oriented transversely to the leaf. The principal (median) 

 bundle also includes many of these elements at its upper end, but has a 

 larger proportion of more elongated tracheal elements in its lower portion 

 (figs. 45, 56). 



AREOLES OR AXILLARY BUDS OF THE OVARY. 



By far the most significant peculiarity in structure of the wall of the ovary 

 in Opuntia fulgida, as compared Avith other angiosperms, is the presence of 

 the axillary buds or areoles distributed over its surface. There is one of 

 these within the leaf, or its scar, at the top of each tubercle, though the basal 

 ones of each fruit remain very rudimentary and never, as far as discovered, 

 give rise to any structures other than a few small spicules or an occasional 

 adventitious root on fruits fallen to the ground. ^Vllat makes these axillarv 

 buds or areoles of the fruits of prime significance in the life of the plant is 

 the fact that those at least of the upper two-thirds of the fruit remain active 

 and each capable of giving rise, on the attached fruit, to a secondary flower 

 or fruit. Or, if the fruit be detached from the plant, these areoles may give 

 rise to adventitious roots and to vegetative shoots, thus initiating new plants. 



