204 



PLANT STRUCTURES 



sac (Fig. 175, at left). Each of these nuclei dhide (Fig. 

 175, at right), and two nuclei appear at each end of the 

 sac (Fig. 175, at middle). Each of the four nuclei divide 



FIG. 175. Lilium Phtiadelphicum : to the left is an embryo-sac with a gametophyte 

 nucleus in each end; to the right these two nuclei are dividing to form the two 

 nuclei shown in each end of the sac in the middle figure. CALDWELL. 



(Fig. 176, at left), and four nuclei appear at each end (Fig. 

 176, at middle). When eight nuclei have appeared, nuclear 

 division stops. Then a remarkable phenomenon occurs. 

 One nucleus from each end, the two being called "polar 

 nuclei," moves toward the center of the sac, the two meet 

 and fuse (Fig. 176, at right, C), and a single large nucleus 

 is the result. 



The three nuclei at the end of the sac nearest the micro- 

 pyle are organized into cells, each being definitely sur- 

 rounded by cytoplasm, but there is no wall and the cells 

 remain naked but distinct. These three cells constitute 

 the egg-apparatus (Fig. 176, at right, A), the central one, 

 which usually hangs lower in the sac than the others, being 

 the egg, the two others being the synergids, or "helpers." 

 Here, therefore, is an egg without an archegonium, a dis- 

 tinguishing feature of Angiosperms. 



