5 6 4 
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 diffu¬ 
sion and great specialization of this interrelation, and to explain the causal 
factors in determining the marvelous phenomena attending its development. 
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. 
Fertilization 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 ovary 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., embryo-sac; e.c., egg-cell; s.n., 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 and egg-nucleus. (After 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 applying 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- 
