23C PLANT HYBRIDIZATION BEFORE MENDEL 



either from having been exposed to different conditions, or to their hav- 

 ing varied from unknown causes in a manner which we in our ignor- 

 ance are forced to speak of as spontaneous." (lb, pp. 302-3.) 



Darwin's experiments indicated, as in the case of heartsease 

 and sweet peas, that 



". . . the advantage derived from a cross between two plants was not 

 confined to the offspring of the first generation." (lb, p. 305.) 



"Laxton's varieties [of sweet peas] produced by artificial crosses," as 

 Darwin says, "have retained their astonishing vigour and luxuriance for 

 a considerable number of generations." {ib., p. 305.) 



Darwin concludes : 



"As the advantage from a cross depends on the plants which are 

 crossed differing somewhat in constitution, it may be inferred as prob- 

 able that under similar conditions a cross between the nearest relations 

 would not benefit the offspring so much as one between non-related 

 plants." {ib., p. 305.) 



Darwin finally also remarks in general : 



"it is interesting to observe . . . the graduated series from plants 

 which, when fertilized by their own pollen, yield the full number of seeds, 

 but with the seedlings a little dwarfed in stature, to plants which, when 

 self-fertilized, yield few seeds, to those which yield none, but have their 

 ovaria somewhat developed, and, lastly, to those in which the plant's 

 own pollen and stigma mutually act on one another like poison." (ic, 

 2: 119.) 



The relative weight and germinative energy of seeds from 

 crossed and from self-fertilized plants, was investigated by Dar- 

 win in the case of sixteen species, with the result that the weight 

 of the seeds of the former to that of the latter was found on 

 the average to be as 100:96. In ten out of the sixteen cases, the 

 self-fertilized seeds were either equal or superior to the crossed 

 in weight, and in six out of these ten, the plants raised from these 

 selfed seeds were greatly superior in height and in other respects 

 to those from the crossed seeds. In the matter of the germination 

 of selfed and crossed seeds, the results were conflicting. Darwin, 

 however, discovered that, in general, seedlings of greater con- 

 stitutional vigor are obtained when crossed by other individuals 

 of the same stock than when pollinated by their own pollen. 



In plants of fifty-seven different species belonging in all to 

 fifty-two genera and to thirty different families, Darwin carried 

 out the most extensive experiment yet on record, conducted for 

 the purpose of determining the difference in size between the off- 

 spring of cross-fertilized and of close-fertilized plants. 



