CROSS POLLINATION 357 



of the flower and their stigmas are ripe (incomplete 

 protandry). The same insect alighting on the centre 

 of the flower in this (the " female ") stage will brush 

 off on the stigmas any pollen that may be adhering to 

 the lower side of its body from a previous visit to a 

 flower in the male stage, thus effecting cross-pollina- 

 tion. A small insect, on the other hand, may alight 

 anywhere, on the petals or among the anthers, and 

 will crawl down to the nectary or wander about eating 

 pollen, and it will only touch the stigmas in the female 

 stage by chance, if at all. The majority of large con- 

 spicuous flowers are pollinated mainly by large insects. 

 In the absence of insect visitors, which do not visit 

 flowers in cold dull weather, the stigmas of the butter- 

 cup eventually curl out far enough to come into 

 contact with the innermost anthers, which still probably 

 have some pollen adhering to them, and thus effect 

 self-pollination. 



In the corncockle {Lychnis githago) and the pinks 

 (Dianthus) protandry is complete, so that self-pollina- 

 tion is impossible. The anthers lie on the platform 

 provided by the flat limbs of the petals on which the 

 insect must alight. At this time the unripe stigmas 

 are concealed in the narrow tube of the flower. Then 

 the anthers fall off and the stigmas grow up and lie 

 on the platform in exactly the same position that the 

 anthers previously occupied. 



In other flowers which are not protandrous the stigmas 

 are generally held well in advance of the anthers, so 

 that they are the first objects the insect meets in entering 

 the flower, and any pollen it already bears will tend to 

 be rubbed off on them. On pushing further into 

 the flower the anthers are encountered. 



Some flowers are so nicely adjusted to the structure 



