SEED-BEARING PLANTS 



441 



393. Fertilization and Formation of Embryo. After 

 pollination the pollen-grain, stimulated by the sticky 

 substance secreted by the stigma, begins to develop a 

 pollen-tube, which passes down the canal that extends 

 through the style from stigma to placenta, nourished by 

 the specialized cells that line the inner surface of the 



FIG. 327. Lilium Martagon. Longitudinal section of the stigma and 

 upper part of the style. The pollen-grains, caught on the papillse of the 

 stigma, have germinated, and the pollen-tubes are growing down along 

 the walls of the style-canal, nourished by the specialized cells that line it. 

 (After Dodel-Port.) 



canal (Cf. Figs. 324 and 327). When the tip of the tube 

 has passed through the micropyle and into the embryo-sac, 

 the sperm-cells, formed during the growth of the tube, pass 

 out into the protoplasm of the embryo-sac, and one of 

 them fuses with the egg-cell, thereby accomplishing fer- 

 tilization (Fig. 328. Cf. Fig. 325). In some cases the 

 embryonic tissue that arises from the fertilized egg gives 



