Cross-fertihsahon 



131 



In many cases it is impossible for the flowers to be self- 

 fertilised, as either the anthers are ripened before the stigmas, 

 or the stigmas before the anthers. This is known as dicho- 

 gamy, and in the former case the flower is protaudrous, and 

 the pollen has to fertilise the pistil of a flower which preceded 

 it in opening ; whilst in the latter case the flowers are protogy- 

 nous, and the pollen fertilises a later flower. 



A good example of a protandrous plant, and one which at 

 the same time shows mechanical arrangements for ensuring the 

 delivery of the pollen upon the right part of the insect's body, 

 is to be found in the Sage. Fig. 252 shows in diagrammatic 

 form the arrangement of the parts in Salvia officinalis. The 

 corolla is bilabiate, the lower lip forming a convenient resting- 

 place for the insect to stand upon, whilst the upper lip protects 

 the stamens and pistil from the rain. 



The stamens, two in number, have branched connectives ; 

 the lower lobes are abortive and united together (b), so that if 

 either be pushed it affects both 

 stamens. The upper lobe is full 

 of pollen, and is poised upon 

 the movable connective in such 

 a manner that if the abortive 

 lobes are pushed backwards and 

 upwards the upper ones come 

 downwards and forwards. In 



Fig. 252. —Sage fertilisation. 



the corolla tube honey is secreted. The style and stigmas 

 at first are placed well back against the upper lip, and are not 

 matured until after the pollen. When they are matured, how- 

 ever, the style bends forward (d), bringing the stigmas near the 



