32 THE FRUIT OF OPUNTIA FULGIDA. 



and notliing comes of it. If it drops on moderately m.oist soil, then it may 

 give rise to roots and shoots by proliferation from the areoles. More rarely 

 still, the fruit may set the seeds free by decay and the latter may possibly 

 then germinate to seedling plants. This latter fate, which must be regarded 

 as the most normal one for such a fruit, is evidently a very rare one for the 

 fruit of Opuntia fulgida, at least in the deserts near Tucson, where seedlings 

 have never been seen. 



THE SEED: ITS STRUCTURE, PERSISTENCE, AND 



GERMINATION. 



The ripe seed is the one essential structure of the fruit which remains 

 entirely unchanged year after year, as the fruit persists on the parent plant. 



The mature or ripe seed is an irregular, round-angled, flattened disk about 

 5 mm. in diameter and d.5 to 2 mm. thick in the middle (fig. 77). It usually 

 bulges on both sides of the disk, but often far more on one side than the 

 other. The majority of good seeds are pale yellow in color, with remnants 

 of many colorless cells from the fleshy ovule-stalks sticking to them. In 

 internal structure the seed consists of a well-developed curved embryo bent 

 in the plane of the disk-like seed, of a small remnant of endosperm near the 

 center of the seed and embraced by the bent embryo, and finally of a protect- 

 ing jacket, formed chiefly by the layers of tissue arising from the pocket of 

 the funiculus, but including also the two thin integuments (figs. 76, 77). 



The embryo is from 0.7 to 1 mm. in diameter and about 4 or 5 mm. long. 

 It is bent in the plane perpendicular to that of the adjacent faces of the coty- 

 ledons. Most of the cells of both cotyledons and radicle of the embryo are 

 densely stored with what appear to be aleurone grains and slime globules. 

 These bodies take on a brownish and not a bluish color with iodine, and 

 show structural features not characteristic of starch-grains. Occasional 

 cells of the cotyledons and upper part of the radicle are completely occupied 

 each wath a large crystal of calcium oxylate like those found in the paren- 

 chyma of the mature plant. 



The endosperm, which consists of a small mass in the bend between radicle 

 and cotyledons and of thinner layers extending along beside the radicle and 

 over the tips of the cotyledons (fig. 98), is densely stored with starch which 

 reacts in the usual way with iodine. 



Small fragments of perispemi, left in the corners where the embryo fits 

 the integument less accurately, are filled with granules reacting like those in 

 the embryo itself. The cell-contents of both embryo and endosperm seem to 

 remain entirely unchanged in fruits that have persisted on the plant for 

 many years after the maturing of the seeds. The integuments are made up 

 of considerably thickened cells, the outermost layer of them with wavy 

 broA^Ti walls. 



The jacket derived from the funiculus surrounds the seed completely, 

 having finally closed in above the micropyle (fig. 97). It makes up more 



