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 egg gives 
