108 



CAUSES WHICH CHECK 



Chap. XVI. 



■ 

 ■ 



s 



g 



of each variety was prepotent on its own stigma oyer that of all other 

 varieties. But 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 flower 

 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 propagatin 

 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 have not been injured by, castra- 

 tion. Girou de Buzareingues crossed what he designates three varieties 

 of gourd, 19 and asserts that their mutual fertilisation is less easy in propor- 

 tion to the difference which they present. I am aware how imperfectly the 

 forms in this group were until recently known ; but Sageret, 20 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.- 1 

 Sageret 22 has observed that certain melons have a greater tendency, what- 

 ever 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 Cit- 

 rullus, 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 it is 



highly remarkable, and is established on excellent evidence. Kolreuter 

 minutely describes five varieties of the common tobacco, 23 which were 

 reciprocally crossed, and the offspring were intermediate in character and 

 as fertile as their parents : from this fact Kolreuter inferred that they are 

 really varieties ; and no one, as far as I can discover, seems to have doubted 

 that such is the case. He also crossed reciprocally these five varieties with 

 N. ghitinosa, and they yielded very sterile hybrids ; but those raised from 

 the var. perennis, whether used as the father or mother plant, were not so 

 sterile as the hybrids from the four other varieties. 24 So that the sexual 



19 



Namely, Barbarines, Pastissons, (3) Transylvanica ; (4) a sub-var. of 



Giraumous : ' Annal. des Sc. Nat.,' torn. the last ; (5) major latifol. fl. alb. 



24 Kolreuter was so much struck with 



this fact that he suspected that a little 



pollen of N. glutinosa in one of his ex- 



21 ' Annales des Sc. Nat./ 4th series, periments might have accidentally got 



xxx., 1833, pp. 398 and 405. 



20 'Memoire sur les Cucurbitacese,' 

 182G, pp. 46, 55. 



torn. vi. M. Naudin considers these 

 forms as undoubtedly varieties of Cu- 

 curbita pepo. 



22 ' Mem. Cucurb./ p. 8. 



23 ' Zweite Forts.,' s. 53, namely, Ni- 



mingled with that of var . perennis, and 

 thus aided its fertilising power. But 

 we now know conclusively from Gartner 

 (' Bastarderz.,' s. 34, 43) that two kinds 

 of pollen never act conjointly on a third 





eotiana major vulgaris ; (2) perennis ; species ; still less will the pollen of a 





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