ANGIOSPERM^E 



179 



Pt 



spore or embryo-sac (Fig. 44, A, ma, B), which in its 

 origin corresponds closely to that in the Gymnosperms 

 and in such Pteri- 

 dophytes as Isoetes. 

 As in the Gymno- 

 sperms, the macro- 

 spore remains per- 

 manently within 

 the ovule. The 

 gametophy te is usu- 

 ally extremely re- 

 duced, showing in 

 the typical forms 

 a very constant 

 structure. 



The single nu- 

 cleus of the ma- 

 crospore divides, 

 and one of the two 

 resulting nuclei 

 moves to each end 

 of the spore-cavity 

 or embryo-sac. 

 Here each nucleus 

 divides twice, so 

 that there result 

 four nuclei at each 

 end of the sac. 

 Three of them re- 

 main at the ends, 



while the fourth one from each end moves toward the 

 centre of the embryo-sac where these two " polar nuclei ' 



FIG. 44. A, diagram showing the arrange- 

 ment of parts in a typical angiospermous 

 flower ; c, the calyx made up of individual 

 leaves, sepals ; co, the corolla, composed of 

 petals; an, the androacium, composed of 

 the stamens : </?/, the gynoecium, made up 

 of the carpels (here but a single carpel). 

 The upper part, anther, an, of each stamen 

 has usually four microsporangia, or pollen- 

 sacs; the macrosporangia, or ovules, are 

 contained in the ovary, formed by the base 

 of the carpel, or united carpels ; p, a pol- 

 len-spore germinating upon the stigma, and 

 sending its tube, pt, down through the cen- 

 tral part of the gynoecium or pistil ; B, dia- 

 gram showing the structures within the 

 embryo-sac at the time of fertilization. 

 The three cells at the upper end form the 

 "egg-apparatus," consisting of the two 

 syuergids, sy, and the egg-cell, o. At the 

 lower end are the three "antipodal-cells," 

 ant, and in the centre are the two "polar 

 nuclei," pn, which afterward unite into a 

 single one, the "endosperm-nucleus"; C, 

 young pollen-spore of Naias, showing the 

 antheridial cell, x ; D, a germinating pollen- 

 spore of the sweet-pea ; pt, pollen-tube ; x, 

 sperm-nucleus. 



