THE FETJIT OF OPUNTIA FULGIDA. 15 



The youngest stage of the nectary seen was a low, conical projection of the 

 superficial layers of the meristem among the trichomes immediately beside 

 (abaxial to) the growing-point of the areole (figs. 12, 50). Very soon a 

 small vascular strand is differentiated, just below the base of the nectary, 

 which later penetrates a short distance into it (figs. 50, 56). As the nec- 

 tary grows it widens somewhat near the top and becomes more or less con- 

 stricted at the base (fig. lY). "When mature, the stalk of the nectary has 

 about three-fifths the diameter of its upper half. The top usually has a 

 small depression with a small tubercle at its center (figs. 17, 20). Some- 

 times this tubercle develops to a well-marked spine (fig. 23). These facts 

 clearly indicate, as has been noted by Ganong (1894, p. 59), that the nectary 

 is homologous with the spines that are so abundant on the joints of many 

 opuntias, but are often wanting on their fruits. The steady increase in 

 number of the nectaries is therefore strictly comparable with the constant 

 increase in number of the spines of the areole of the vegetative joint 

 (c/.p. 8). 



The epidermis of the mature nectary is small-celled and thin-walled, 

 except at the top and on the spine or tubercle itself. Here the cells are small 

 and irregularly compacted and their walls are provided with a cutin layer 

 several microns in thickness, which peels off rather readily (fig. 56). The 

 cells of the interior of the nectary seem to be little differentiated, except for 

 the occasional trace of a vascular bundle near the base. They are often 

 longitudinally elongated to 10 or 12 times their diameter and are commonly 

 pointed at both ends (fig. 56). Most of these cells in the mature nectary 

 have darkly staining protoplasts and nuclei, but a considerable number of 

 them are nearly devoid of contents except for a large, stellate crystal of 

 calcium oxalate. 



The mature nectary remains plump and probably active for one growing 

 season and then, as has been suggested, it gradually withers and dries up to a 

 shriveled brown rod of scarcely a tenth the diameter of the functional nec- 

 tary. It is this shriveled mummy that persists indefinitely, year after year, 

 among the trichomes of the areole (fig. 14) . 



SPINES OR THORNS OF THE AREOLE : THEIR DISTRIBUTION, STRUCTURE. AND FATE. 



While spines, to the number of 6 or 8 and of a length of 3 or 4 cm., are 

 present in areoles of the vegetative joint of Op^mtia fulgida, they are usually 

 wanting from the fruits {cf. Wetterwald, 1889, fig. 18). When spines do 

 occur in a fruit there is usually but a single spine in each of only a few of 

 its areoles (figs, n, 1c). The position of this spine is essentially that of the 

 first nectary of the more normal fruits ; that is, it stands in the sagittal plane 

 of the areole, just within the subtending leaf. 



The structure of these spines may be described briefly here, leaving the 

 fuller discussion of this and their development for a later paper in which it is 

 planned to deal more in detail with the stem. The spines initiated on the 

 fruit seldom attain the size and strength of those on the stem. They also 



