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 papillae 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 tgg gives 



