8 THE FRUIT OF OPUNTIA FULGIDA. 



VEGETATIVE STRUCTURE OF OPUNTIA FULGIDA. 



Opuntia fulgida is a tree-like, Sonoran species of Cylindropmvtia, which 

 commonlj grows to 2 or 3 meters in height and forms a rather irregular flat- 

 topped crown. The older or main branches are horizontal, or ascendant at 

 their bases, but bent do^vn at their tips by the weight of the thick terminal 

 branchlets and often of the large clusters of fruits (fig. 1). The spiny 

 trunk is dark brown in color, woody in texture, and may reach 20 or 25 cm. 

 in diameter. The main branches, which are woody like the trunk, may 

 become 8 or 10 cm. in diameter. The ultimate branches at the end of the 

 first season's growth are often 3 to 5 cm. in diameter and 15 to 25 cm. 

 long. These younger branches taper abruptly at the ends and the lateral 

 surfaces are provided with prominent and somewhat elongated mammillae 

 or tubercles (figs. 4, 5, 9a). Each tubercle, at this time, bears at its upper 

 end from 7 to 12 sheathed spines and the growing-point of a lateral bud 

 (figs. 4, 95). The number of spines in each areole steadily increases with 

 age, and hence a branch 10 years old may bear 50 spines in each areole. 

 The leaf to which the areole is axillary is a small and veiy transient structure 

 which (on falling) leaves only a minute scar, like that to be seen on the 

 fruit (figs. 5, 47). The vascular system of the stem is net-like in arrange- 

 ment, being of the same general type as that described and figured by 

 Ganong (1894, fig. 7). During the first year the bulk of the new joint is 

 made up of the mucilaginous pulp of the pith and cortex. The latter has 

 a well-developed photosynthetic and aerating system of the type to be 

 described in dealing with the fruit, and large numbers of slime-cells {cf. 

 Wetterwald, 1889, fig. 19). Chloroplasts are abundant throughout the 

 whole thickness of the cortex of the stem and may even occur within the 

 zone of woody bundles, as happens in the projecting tubercles. With the 

 increase in thickness of the branch the woody cylinder seems to gTOW in 

 diameter more rapidly than the fleshy cortex. The latter finally becomes 

 stretched and smoothed out and the cortex of the mature stem is compara- 

 tively thin and dry (fig. 2, at base). 



On some plants of this species, as has been noted by Toumey (1895), cer- 

 tain of the new joints may remain relatively short and have less prominent 

 tubercles and fewer spines, becoming thus rather fruit-like in form (fig. 7a). 

 These joints are readily detached and on moist soil may give rise to new 

 plants by proliferation, just as the ordinary vegetative branches of this and 

 many other opuntias may do. 



