96 Principles of Plant Culture. 
the stigma by some outside influence, as by insects or the 
wind, or by gravity. Most flowers which have ashowy corolla 
or calyx, or secrete nectar, or yield a fragrant perfume, 
depend largely upon the visits of pollen-loving or nectar- 
loving insects for pollination. The showy parts, and the 
perfume serve as signboards to direct the wandering insects 
to the flowers. 
152. Cross Pollination occurs when the stigma is pol- 
linated with pollen from a different plant, especially from a 
plant of a different variety or species (21). The fecunda- 
tion resulting constitutes a cross or hybrid, as the case may 
be (23). Cross pollination is often performed artificially (441). 
Close- or self pollination occurs when the stigma receives 
pollen from its own flower or plant. | 
153. Cross Pollination is Advantageous in Plants, as 
Darwin’s careful experiments have shown. The seeds 
formed are usually more numerous and larger, and form 
more vigorous plants, than with close pollination. Ks- 
pecially is this true when the parent plants have been sub- 
jected to different growth conditions in previous generations. 
Nature favors cross pollination in perfect-flowered plants 
by numerous adaptations tending to prevent self pollina- 
tion, as maturing the pollen either before or after the recep- 
tive stage of the stigma, or so locating the stamens that the 
pollen is not readily deposited on the stigma of the same 
flower.* In some cases, pollen is infertile on a stigma of the 
same flower or plant that is abundantly fertile + on stigmas of 
other piants of the same species (155). 
* Darwin’s work ‘‘On the Various Contrivances by which Orchids are Fertil- 
ized by Insects” describes many most interesting adaptations of this sort. 
+ Fertile pollen is pollen that is capable of fecundating female cells of its own 
species. 
