546 



THE PURE LINE HYPOTHESIS. 



in the third seven sets of seed, with an average of 22 seeds in the 

 set, the percentage number which germinated was 67%. The 

 average number of seeds which germinated in all the sets was 

 55%. 



With seed obtained by cross-fertilisation and with as careful 

 sowing there is evidence that at least about 80-85% would 

 germinate. Thus in the first generation of selling there was a 

 reduction of about 30% in the germinating power of the seeds. 



6. The Effect of Hybridism on Fertility. 



The crossing of the two varieties of nasturtium was pre- 

 judicial to the generative system of the hybrids, since it was 

 discovered that a considerable portion of their pollen was in- 

 capable of germinating. 



It is not always easy to determine by inspection whether a 

 pollen grain is bad, and to prevent any possible bias in the 

 estimation, Dr. Conrad Akerman, without any knowledge of the 

 relative fertility of the plants, kindly made counts of the number 

 of good and bad grains in six different microscopic fields of pollen 

 taken from each of the 21 hybrid plants. The percentage of good 

 pollen grains and the number of offspring raised from the 21 

 hybrids is given in the following table : — 



