SPERirATOPHYTES : ANGIOSPEEMS 



203 



its cells but not penetrating them, enters the cavity of the 

 ovary, passes through the micropyle of an ovule, penetrates 

 the tissues of the nucellus (if any), and finally reaches and 

 pierces the wall of the embryo-sac, within which is the egg 

 awaiting fertilization (Fig. 173). 



This remarkable ability of the pollen-tube to make its 

 way through so much tissue, directly to the micropyle of 

 an inclosed ovule, can only be explained by supposing that 

 it is under the guidance of some strong attraction. 



113. The female gametophyte. — The megaspore (embryo- 

 sac) occupies the same position in the ovule as in Gymno- 

 sperms, but its germination is remarkably modified. The 

 development of the female gametophyte shows two distinct 



Fig. 174. Zilium Philadelphicum : to the left a young megaeporangium (ovnle), 

 showing integuments ( 6'). nucellus (A), and megaspore (B) containing a large nu- 

 cleus. To the right a megaspore whose nucleus is undergoing the first division 

 in the formation of the gametophyte. — Caldwell. 



periods, separated from one another by the act of fertiliza- 

 tion. If fertilization is not accomplished the second stage 

 of the gametophyte is usually not developed. 



Mrst period. — The megaspore nucleus divides (Fig. 

 174), and one nucleus passes to each end of the embryo- 



