INSECTS AND FLOWERS 



451 



describe in wonderful detail this mutually advantageous 

 interrelation between flowers and insects and to explain 

 its chief causal factors. These are, first, the real advantage 

 to the plant of cross-poUination, and, second, the action of 

 natural selection in modifying both flowers and insects for 

 the sake, or by the reason, of this advantage. 

 Fertilization among plants is like fertilization among 



Fig. 235. Diagram of section of pistil and ovary of a flower, showing the 

 descent of the pollen tube and its entrance into the ovule, f. g., pollen- 

 grain; p. t., pollen-tube; e. s., embryo sac; e. c, egg-cell; s. »., sperm 

 nucleus. Left-hand figure (1) shows the pollen-tube grown down, 

 around and up into the ovary with the sperm-nucleus just entering the 

 ovule; right-hand figure (2) shows the fusion of the sperm-nucleus. 

 (After Stevens.) 



animals; a germ- (sperm-) cell from one individual (male or 

 hermaphrodite) fuses with a germ- (egg-) cell from another 

 (female or hermaphrodite) individual or from the same 

 (hermaphrodite) individual. The sperm-cells are contained 

 in pollen produced in the anthers of stamens; the egg-cells 

 lie in the ovaries at the base of the pistils, these pistils having 

 an exposed pollen-catching surface (stigma) at their free 

 tip. Before actual fertilization can occur pollination roust 



