in.] THE FERTILISATION OF FLOWERS. 47 



pollen is all exhausted the stigma is protruded in the 

 very same place, so that if it should press against a 

 bee which has thus obtained pollen from a younger 

 flower, cross-fertilisation is certain. The mechanism 

 by which this is effected may be easily seen if any 

 papilionaceous flower be taken and gentle pressure 

 exerted on the wings. The tip of the keel will be 

 exposed and a thin curl of pollen forced out, just as 

 it would be against the bee's body. 



Every one is familiar with the flowers of Trop&olum 

 major ) better known as the Nasturtium. The corolla 

 is continued backwards as a long hollow spur which 

 contains the honey. When the flower first opens all 

 the organs are immature, but soon an anther becomes 

 mature and erects itself in front of the entrance to the 

 spur, so that no bee could get at the honey without 

 pushing against the anther. All the other stamens 

 erect themselves in like manner, one at the time, until 

 all the pollen is shed, when they hang down out of 

 the way, and the pistil, which has been slowly matur- 

 ing and lengthening, raises itself to the position 

 previously occupied by the stamens. Insects which 

 have visited young flowers cannot help fertilising 

 older ones when they visit them. Self-fertilisation 

 cannot take place. 



The Primrose, Cowslip, &c., are well known to 

 have two forms of flowers. Even the children are 

 acquainted with this peculiarity of the genus, and 

 call them " thrum-eyed " and " pin-eyed," according as 

 the stigma or stamens are most prominent. In the 

 one form (fig. 56) the pistil is elongated and the 

 stigma on a level with the top of the corolla-tube. In 



