CHAP. V. PISTJM SATIVUM. 161 



suppose that the fertilisation of these flowers is effected by the 

 carriage of pollen from one to the other." Notwithstanding these 

 manifest provisions for cross-fertilisation, varieties which have 

 been cultivated for very many successive generations in close 

 proximity, although flowering at the same time, remain pure. I 

 have elsewhere * given evidence on this head, and if required 

 could give. more. There can hardly be a doubt that some of 

 Knight's varieties, which we/re originally produced by an artificial 

 cross and were very vigorous, lasted for at least sixty years, and 

 during all these years were self-fertilised ; for had it been other- 

 wise, they would not have kept true, as the several varieties 

 are generally grown near together. Most of the varieties, how- 

 over, endure for a shorter period ; and this may be in part due 

 to their weakness of constitution from long-continued self- 

 fertilisation. 



It is remarkable, considering that the flowers secrete much 

 nectar and afford much pollen, how seldom they are visited by 

 insects either in England or, as H. Muller remarks, in North 

 Germany. I have observed the flowers for the last thirty years, 

 and in aU this time have only thrice seen bees of the proper kind at 

 work (one of them being Bombus muscorum), such as were suf- 

 ficiently powerful to depress the keel, so as to get the undersides ot 

 their bodies dusted with pollen. These bees visited several flowers, 

 and could hardly have failed to cross-fertilise them. Hive-bees 

 and other small kinds sometimes collect pollen from old and 

 already fertilised flowers, but this is of no account. The rarity 

 of the visits of efficient bees to this exotic plant is, I believe, the 

 chief cause of the varieties so seldom intercrossing. That a 

 cross does occasionally take place, as might be expected from what 

 has just been stated, is certain, from the recorded cases of the direct 

 action of the pollen of one variety on the seed- coats of another, f 

 The late Mr. Masters, who particularly attended to the raising of 

 new varieties of peas, was convinced that some of them had 

 originated from accidental crosses. But as such crosses are 

 rare, the old varieties would not often be thus deteriorated, more 

 especially as plants departing from the proper type are generally 

 rejected by those who collect seed for sale. There is another 

 cause which probably tends to render cross-fertilisation rare; 



* 'Variation of Animals and f 'Var. under Domestication, 

 Plants under Domestication,' chap. xi. 2nd edit. vol. i. p. 428. 

 chap. ix. 2nd edit. vol. i. p. 3-48. 



