FERTILISATION EFFECTED BY MEANS OF INSECTS. 287 



cession towards the pistil, and after the pollen has been discharged 

 they curve back to the petals. But the stigma is not perfect at that 

 time. It becomes developed after the pollen has been discharged and 

 the anthers have retired. It requires the agency of insects to effect 

 complete fertilisation. The pollen is discharged on the part visited 

 by insects, and they take it up on that part of their bodies which 

 touches the perfect stigma in other flowers, and thus fertilisation is 

 effected. In Lobelia we have an instance of the stamens being com- 

 plete and the pollen discharged before the stigma is perfect. After 

 the poUen has been discharged, the style elongates and carries the 

 stigma upwards beyond the syngenesious anthers, and then the stigma 

 becomes perfect, so as to be ready for the pollen applied by insects. 

 Both these flowers are Protandrous. 



In Euphorbia jacquiniflora, several days before the stamens burst 

 through the involucre which closely invests them, the pistil with its 

 ovary on the long pedicel has protruded itself beyond, expanded its 

 stigma, and received pollen from neighbouring flowers. It is there- 

 fore Protogynous. 



In the case of Aristolochia Clematitis (fig. 515), the flowers, as 

 long as the essential 

 organs are in a state 

 fit for fertilisation, 

 stand erect, with their 

 oblique mouth turned 

 outwards, by which an 

 insect can enter easily, 

 and pass down the tube 

 till it comes to the 

 column bearing the 

 stamens and stigma. 

 It is prevented from 

 returning by inverted 

 hairs in the tube. It 

 is detained in the tube 

 tiU the pollen is fully 

 matured, and then the 

 hairs collapse so as to 

 permit its escape. It 

 carries with it pollen 

 grains. It then visits 

 a flower where the 

 stigma is matured, and 

 which presents the open 

 mouth of the tube in an erect condition, and on reaching the cavity 



Mg. 615. Jflowering stalk of Common Birthwort (AristolocMa Clematitis}. Fertilisation 

 is effected by insects. 



Fig. 615. 



