564 



Insects and Flowers 



merely of his acquaintance with the observations of Sprengel, Waechter, 

 Delpino, Hooker, and others, but of characteristically keen and careful 

 investigations of his own (particularly on orchids) to reveal the wide difTu- 

 sion and great specialization of this interrelation, and to explain the causal 

 factors in determining the marvelous phenomena attending its develoj)ment. 

 These causal factors are (i) the real advantage to the plant species of cross- 

 fertilization, and (2) the action of natural selection in modifying both flowers 

 and insects for the sake, or by reason of, this advantage. 



Fertihzation among plants is like fertilization among 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 



Fig. 761. — Diagram of section of pistil and ovarj- of a flower, showing the descent of 

 the pollen-tube and its entrance into the ovule, p^g-, pollen-grain; p.t., pollen- 

 tube; e.s., cmbrjo-sac; ex., egg-cell; s.n., sperm-nucleus. Left-hand figure (i) 

 shows the pollen-tube grown down around and up into the ovarj' with the sperm- 

 nucleus just entering the ovule; right-hand figure (2) shows the fusion of the 

 sperm-nucleus and egg-nucleus, (.\fter Stevens.) 



base of the pistils, these pistils having an exposed pollen-catching surface 

 (stigma) at their free tip. Before actual fertilization can occur pollination 

 must take place; pollination being the bringing and ai)plying of ripe pollen- 

 grains to the ripe surface of the stigma. How fertilization then takes place 

 is succinctly explained by Fig. 761 and its caption, which is copied from 

 Stevens (Introduction to Botany, Boston, 1902). 



Cross-pollination is simply the bringing of pollen from one plant indi- 

 vidual to the stigmas of another individual of the same species. Self-pol- 



