SEXUAL AFFINITY 285 



(2) Powdery, dry, character of the pollen . 



(3) The extensive receptive surface of the stigmas. 



(4) General absence of scent and nectar. 



Ex. 171. — Examine the following insect-pollinated flowers :— buttercup, 

 columbine, monk's-hood, poppy, cabbage, pansy, violet, pink, carnation, 

 primrose, slitchworts, mallows, horse-chestnut, bean, clovers, birds' foot 

 trefoil and other leguminous plants, strawberry, apple, pear, cherry, plum, 

 dandelion, sunflower, thistle, knapweed, parsnip, carrot and other um- 

 belliferous plants. 



Make an examination of the flowers in different states of development, and 

 note : — 



(i) Whether they are protogynous or protandrous. 



(2) Where the nectar is secreted and stored if any is present : it may be at 

 the base of the stamens, on the receptacle, ovary, or in specially constructed 

 parts of the petals and sepals. Frequently ridges and variegated stripes of 

 colour on the petals point in the direction of the nectary, and apparently 

 serve as guides to insect-visitors. 



(3) Determine whether there is any specially convenient landing-place for 

 insect-visitors, and try and find out whether the stigma or anthers are touched 

 first when insects visit the flowers. 



(4) Whenever opportunity offers, carefully watch insects at work extract- 

 ing honey or collecting pollen from flowers. 



6. Sexual af5.mty : hybridisation and hybrids. — A fertile 

 sexual union between the male reproductive cell of a pollen-grain 

 and the egg-cell within an ovule does not take place indiscrimin- 

 ately among plants. A certain relationship or sexual affinity 

 must exist between the parent plants before their reproductive 

 cells will unite. 



Although self-fertilisation is possible, and among certain plants 

 is a normal process, experience teaches that in many cases 

 the pollen of a flower has no fertilising effect on the egg-cells 

 of ovules present in the same flower or in flowers on the same 

 individual plant. 



Moreover, it is generally found that fertilisation between the 

 reproductive cells of plants widely different from each other, say, 

 between those of a cabbage and a potato, or those of a peach 

 and a turnip, does not take place. 



