INTRODUCTION aI 
The flower is conspicuous in order to attract 
insects, or its attractiveness we may say results in 
the visits of insects. 
The occurrence of male and female flowers on 
different plants is of course again a device for pre- 
venting autogamy or self-pollination. In order to 
effect cross-pollination, which is necessary in this 
case, insects must transfer pollen from one plant to 
the stigma of another. 
Another device is the ripening of flowers, where 
male and female are on the same plant, of the 
anthers and stigma at different times (dichogamy). 
In the one case the anthers may ripen first, when the 
flower is proterandrous, in the other the stigma may 
do so, when the flower is proterogynous. 
If in such flowers both ripened together in the 
absence of insect visits there would be self-pollina- 
tion unless the anthers were so placed that a visit by 
insects was necessary, or the anthers were turned 
away from the stigmas when ripe. Even if insects 
visit such flowers when the female and male elements 
were ripe together (homogamous) there might be 
self-pollination again unless the mechanism of the 
flower prevented this. If the male and female 
flowers are on the same plant but not in the same 
flower there is more chance of cross-pollination than 
if they are on different plants, as so much more 
pollen is required for the certain crossing of male 
and female flowers. 
Some plants, again, have more than one type of 
