198 THE STRUCTUEE OF FLOWERS, 



CHAPTER XXII. 



Hetekogamy* and Autogamy. 



Pkotandry, Protogyny, Homogamy, and Cleistogamy. — These 

 conditions prevail in nature in varying degrees of frequency. 

 The first is common to all conspicuous flowers habitually 

 visited by insects, and is accompanied by heterogamy. The 

 fact that anthers mature their pollen before the stigmas of 

 the same flower are ready to I'eceive it, is due to the extra 

 stimulus given to the androecium, which mostly effects 

 simultaneously the enhancement of the corolla or perianth 

 which attracts the insects (see p. 191). Like everything else 

 in nature, it is very far from being absolute, and any flower 

 may be protandrous at one time or place, while it may at 

 another mature the essential organs together, and then it 

 becomes homogamous, or it may be even protogynous. 



These latter conditions prevail in less conspicuous flowers 

 and all those which are fluctuating between a condition 



* Heterogamy, i.e. union by intercrossing different flowers. 



Autogamy, i.e. union by self-fertilising one and the same flower. 



Protandry, i.e. stamens maturing the pollen before the stigmas of 

 one and the same flower are ready to receive it. 



Protogyny, i.e. pistil maturing the stigmas before the pollen of one 

 and the same flower is shed. 



Homogamy, i.e. pollen and stigmas of one and the same flower 

 maturing simultaneously. 



Cleistogamy, i.e. autogamous within an unopened perianth. 



