Insects and Flowers 
565 
lination is the getting of pollen from the stamens of one flower onto the 
stigma of the same flower. The advantage of cross-pollination, as first experi¬ 
mentally proved by Darwin, and since then confirmed by other experimenters 
and, without scientific intention but none the less effectively, by hosts of 
■economic plant-breeders (horticulturists, florists, etc.), lies in the fact that 
the seeds produced when the ovules of one plant are fertilized by the sperm- 
cells (in the pollen) of another develop plant individuals of markedly stronger 
growth (shown in size of plant and its fruits, in number of seeds, etc.) than 
seeds produced by the fertilization of ovules by sperm-cells of the same plant. 
To effect this advantageous cross-pollination two lines of specialization or 
modification of floral structures have arisen (presumably through the action 
of natural selection): (1) modifications such as to attract insects and insure 
cross-pollination as the result of their visits (and to much less extent to attract 
other animals, particularly humming-birds), and (2) modifications tending 
to prevent self-pollination. Coupled with both these general lines of modi¬ 
fication are others to effect certain auxiliary or accessory conditions the 
necessity for which grows out of the larger needs; such are, for example, 
modifications to prevent the stealing of nectar and pollen by other animals 
(insects particularly) than those on which cross-pollination specially depends, 
and to make possible self-pollination in cases where cross-pollination, 
although probable, may for some accidental or other rare cause not take 
place. Coincidently, and reciprocally with the development of modifications 
of the flower structures, has occurred the specialization of certain structures 
and habits among those insects which are the cross-pollinating agents. 
These modifications occur chiefly in the structure of the mouth-parts and 
legs of bees, wasps, flies, and a few other insects and in their food and 
flight habits, and the care of their young. The reciprocal modifications 
of flowers and insects have gone so far in some cases that certain species of 
plants and certain species of insects cannot now live except by virtue of 
their inter-relation. Many flowers are not fertile when pollinated by their 
own pollen, and yet have no other possible means of getting pollen from 
other plants except that of insect visits. 
The principal means which have been developed to avoid self-fertiliza¬ 
tion are the following: (<z) the having each flower unisexual instead of bi¬ 
sexual, that is, producing either pollen (staminate) or ovules (pistillate) but 
not both; these unisexual flowers may occur on the same plant individual 
(monoecious) or on separate individuals (dioecious); ( b ) the having both 
pistils and stamens on each flower, but with the anthers and the stigma not 
maturing coincidently (dichogamous), either the anthers breaking open 
and discharging the pollen before the stigmas are ready to receive it (pro- 
terandrous) or the stigmas maturing before the pollen ripens and is dis¬ 
charged (proterogynous); (0) the having the stamens and pistils (in the 
