86 CAUSES WHICH CHECK Chap. XVI. 



Canterbury has given me a more striking case ; he saved seed from 

 a great bed of twenty-four named varieties planted in closely ad- 

 joining rows, and each variety reproduced itself truly with only 

 sometimes a shade of difference in tint. Now in the hollyhock the 

 pollen, which is abundant, is matured and nearly all shed before 

 the stigma of the same flower is ready to receive it ; x and as bees 

 covered with pollen incessantly fly from plant to plant, it would 

 appear that adjoining varieties could not escape being crossed. As, 

 however, this does not occur, it appeared to me probable that the 

 pollen of each variety was prepotent on its own stigma over that of 

 all -other varieties, but I have no evidence on this point. Mr. C. 

 Turner of Slough, well known for his success in the cultivation of 

 this plant, informs me that it is the doubleness of the flowers which 

 prevents the bees gaining access to the pollen and stigma ; and he 

 finds that it is difficult even to cross them artificially. Whether 

 this explanation will fully account for varieties in close proximity 

 propagating themselves so truly by seed, I do not know. 



The following cases are worth giving, as they relate to monoecious 

 forms, which do not require, and consequently cannot have been 

 injured by, castration. Girou de Buzareingues crossed what he 

 designates three varieties of gourd, 21 and asserts that their mutual 

 fertilisation is less easy in proportion to the difference which they 

 present. I am aware how imperfectly the forms in this group were 

 until recently known ; but Sageret, 22 who ranked them according to 

 their mutual fertility, considers the three forms above alluded to as 

 varieties, as does a far higher authority, namely, M. Naudin. 23 

 Sageret 24 has observed that certain melons have a greater tendency, 

 whatever the cause may be, to keep true than others ; and M. Naudin, 

 who has had such immense experience in this group, informs me 

 that he believes that certain varieties intercross more readily than 

 others of the same species ; but he has not proved the truth of this 

 conclusion ; the frequent abortion of the- pollen near Paris being 

 one great difficulty. Nevertheless, he has grown close together, 

 during seven years, certain forms of Citrullus, which, as they 

 could be artificially crossed with perfect facility and produced 

 fertile offspring, are ranked as varieties ; but these forms when not 

 artificially crossed kept true. Many other varieties, on the other 

 hand, in the same group cross with such facility, as M. Naudin 

 repeatedly insists, that without being grown far apart they cannot 

 be kept in the least true. 



Another case, though somewhat different, may be here given, as 



20 Kolreuter first observed this fact, 22 ' Memoire sur .es Cucurbitacecc,' 

 * Mem. de l'Acad. de St. Petersburg,' 1826, pp. 46, 55. 



vol. iii. p. 127. See also C. K. 23 'Annales des Sc. Nat.,' 4th 



Sprengel, ' Das Entdeckte Geheimniss,' series, torn. vi. M. Naudin considers 



s. 345. these forms as undoubtedly varieties 



21 Namely, Barbarines, Pastissons, of Cucurbita pepo. 

 Giraumous: ' Annal. des Sc. Nat.' 2i ' Mem. Cucurt.,' p. 8. 

 torn, xxx., 1833, pp. 398 and 405. 



