394 THE BIOLOGY OF FLOWERING PLANTS 



review of the subject, makes a comparison between the 

 inhibition of the pollen tube growth and the phenomena 

 of immunity to disease in the animal kingdom. There is 

 the possibility that in the cells of the stigma an anti-body 

 inhibiting further growth is formed as a reaction to the 

 entrance of the pollen tube. The subject awaits exact 

 investigation. 



§ 15. Self-Pollination 



Cross-pollination is thus widespread amongst flowering 

 plants. Sometimes it is, from one cause or another, obliga- 

 tory ; sometimes it is more or less favoured. Sometimes, 

 however, the chances as between cross- and self-pollination 

 are about equal or inclined to the latter. Self-pollination 

 may be favoured by the structure of the flower, and it, too, 

 may be obligatory. 



Over open flowers like those of the buttercups, poppies, 

 and brambles many insects wander at will, the flowers are 

 homogamous, and, unless self-sterile, as in Ranunculus acris, 

 Papaver Rhoeas, Rubus odoratus, there is nothing to prevent 

 autogamy taking place, and, indeed, it is likely to be the rule. 

 No special mechanism exists, but the chances of indirect 

 autogamy must be strong. Even when special mechanisms 

 which assist cross-pollination exist, there is often a very 

 strong chance of geitonogamy. Bees in particular tend to 

 restrict their visits to a particular flower during considerable 

 periods. Any one who has watched a bee busied about a 

 sage bush knows how other plants are neglected and flower 

 after flower of the sage is tried. Even though the flowers 

 are proterandrous, it is clear that the chances of a stigma 

 being pollinated from another flower on the same plant are 

 great. The same must be true of monocHnous and 

 monoecious wind-pollinated plants, where dichogamy is not 

 complete. In a spike of the ribwort plantain, Plantago 

 lanceolata, the stigmas are produced from the upper flowers 

 while the stamens hang from the lower. In the monoecious 

 proterogynous sedges the stigmas may still be receptive 



