VARIATION OF CHROMOSOMES 155 



while in the latter case, since there are presumably no 

 such types found, there is no conflict. Some other dif- 

 ference would have to be appealed to to explain why the 

 octoploid forms fail to develop. 



A triploid condition {3N) has been found to occur in 

 certain types of the evening primrose (Stomps, Lutz, 

 Gates) . De Vries has found in crosses in which Lamar ck- 

 iana was the mother and some other species (muricata, 

 cruciata, etc.), the father, that triploid types appear three 

 times in 1000 cases. He interprets the results to mean 

 that three in 1000 times the egg-cell of Lamarckiana has 

 the double number of chromosomes (14), which being fer- 

 tilized by a normal pollen grain with seven chromosomes, 

 gives the triploid number, viz., twenty-one chromosomes. 

 The same result would be reached if a diploid pollen grain 

 fertilized a normal egg. That such pollen grains appear 

 is as probable a priori as that diploid eggs occur. It 

 may be recalled that one explanation of the tetraploid 

 evening primrose (gigas) is that it arises from a 2^^^ pollen 

 grain meeting a 2^ egg-cell. How reduction takes place 

 in the triploid Oenotheras is uncertain, since the accounts 

 of the process are different. Greerts states that, as a rule, 

 only seven chromosomes conjugate (7 + 7), while the 

 remaining seven chromosomes are irregularly distrib- 

 uted in the dividing germ-cells. On the other hand, G-ates 

 finds in a 21-chromosome type that the chromosomes 

 separate into groups of 10 and 11, or occasionally into 

 9 and 12. The former account fits in better with results 

 of the same kind obtained by others, and is more easily 

 understood from a general point of view, because seven 

 homologous pairs would correspond to the normal conju- 

 gation, while the seven chromosomes left over would have 

 no mates and fail to divide at the reduction division, hence 

 their erratic distribution. 



It has also been shown in (Enothera that there are 

 three 15-chromosome types. If the 15th chromosome is 



