8 TRANSFERENCE OF POLLEN [CHAP. 



fell off ; the pollen-grains themselves, and the stigma 

 in contact with them, shrivelled up, turned brown, 

 and decayed; while other flowers on the same branch, 

 which were not so treated, retained their freshness. 



The transference of the pollen from one flower to 

 another is, as I have already mentioned, effected 

 principally either by the wind or by insects. In the 

 former case the flower is rarely conspicuous ; indeed 

 Mr. Darwin finds it " an invariable rule that when a 

 flower is fertilised by the wind it never has a gaily- 

 coloured corolla." Conifers, grasses, birches, poplars, 

 &c., belong to this category. 



In such plants a much larger quantity of pollen 

 is required than where fertilisation is effected by 

 insects. Everyone has observed the showers of 

 yellow pollen produced by the Scotch fir. It is 

 an advantage to these plants to flower before the 

 leaves are out, because the latter would greatly in- 

 terfere with the access of the pollen to the female 

 flower. Hence such plants, as a rule, flower early in 

 the spring. Again, in such flowers the filaments of 

 the stamens are generally long, and the pollen is less 

 adherent, so that it can easily be detached by the 

 wind, which would manifestly be a disadvantage in 

 the case of those flowers which are fertilised by 

 insects. On the other hand, it is an advantage to 

 most seeds to be somewhat tightly attached, because 

 they are then only removed by a high wind which is 

 capable of carrying them some distance. I say "to 

 most " because this does not apply to such seeds as 

 those of the dandelion, which are specially adapted 

 to be carried by the wind. 



