358 BOTANY: PRINCIPLES AND PROBLEMS 



Fig. 229. — Development of the female gametophyte and seed in a dicotyledon- 

 ous angiosperm. Longitudinal sections through the ovule and seed. A, very 

 young ovule, the two integuments (1 and 2) beginning to appear and the mega- 

 spore mother-cell evident within the nucellus (3). B, the megaspore mother-cell 

 has produced a row of four megaspores. C, three megaspores have aborted, the 

 fourth is enlarging. D, the megaspore nucleus has divided into two, which now 

 lie at opposite ends of the young embryo-sac (4). E, each nucleus has again 

 divided into two. F, each of the four nuclei has again divided into two, so that 

 there are now two groups of four nuclei, one at each end of the embryo-sac. 

 G, one nucleus from each group has migrated to the center, and the two are 

 uniting to form the endosperm nucleus (6) . The three left at the end of the sac 

 next the micropyle have formed the egg-cell (5) and the two synergid cells. The 

 three at the opposite end have formed the antipodal cells (7) . The embryo sac is 

 now fully developed and the egg is ready for fertilization. H, a pollen tube 

 has entered the micropyle and discharged two male cells into the embryo sac. 

 One of these (8) is uniting with the egg and the other (9) with the endosperm 

 nucleus. /, the fertilized egg has grown into a small embryo (10) and the fertil- 

 ized endosperm nucleus into a sac lined with nuclei (11). /, the mature seed. 

 The integuments have hardened into seed coats, the micropyle is closed, the 

 nucellus has disappeared, the endosperm sac has become a mass of solid endo- 

 sperm, packed with food, and the embryo has reached its full size. 



