ALBERT F. BLAKESLEE 59 



He thought he had discovered visual evidence for the separation of 

 maternal and paternal essences in the Fi generation since in a hybrid 

 between a form with spiny capsules and one with smooth capsules he 

 sometimes found capsules one half of which was smooth and the other 

 half spiny. This is an example of a correct conclusion from faulty in- 

 terpretation, since more than 50 years later we were able to show that 

 Naudin's findings were due to a virus disease which can be propagated 

 by grafting. This environmentally induced type we called "Quercina" 

 from its lyrate leaves. Almost identical effects are produced by a recessive 

 gene which arose from X-ray treatment. DeVries used the segregation 

 of two pairs of characters in Datura in rediscovering the laws of Mendel. 



Of modern students of Datura I want first of all to speak of John 

 Belling, a brilliant pioneer in cytogenetics. His finding that certain 

 mutant types in Datura had single extra chromosomes, which in many 

 cases he could distinguish by size and structure, not only furnished an 

 interpretation of our trisomic ratios from Primary 2n + i types but 

 showed that such types were due to the unbalancing effect of specific 

 chromosomes. This was at the time when some of the leading students 

 of Oenothera believed that extra chromosomes were the effect and not 

 the cause of 2n + i types. It was nearly 20 years after Belling's findings 

 before the successor of de Vries publicly acknowledged that extra 

 chromosomes were the cause of mutant types in the Evening Primrose. 



In another group of 211 + i types which we now call Secondaries, 

 Belling found that the extra chromosome was often associated with 

 two others at metaphase to form a closed trivalent. This could only 

 mean, he believed, that the extra chromosome of the trivalent must be a 

 doubled half chromosome. He was the first to put effectively to work 

 the idea that at metaphase similar chromosome ends are attached like 

 dominos. This interpretation not only explained the morphological 

 pecuUarities and breeding behavior of Secondaries but laid the way 

 for the later discovery of segmental interchange. 



In 1 92 1 there appeared in our cultures a compensating chromosomal 

 type "Nubbin" among the offspring of a plant treated by Gager with 

 radium emanation. This is probably the first induced chromosomal 

 mutation. A couple of gene mutations in the selfed offspring of only 

 18 parents also were found from the same treatment but their number 

 was too small to warrant being sure that their appearance was due to the 

 radiation. Later Muller from his brilliant work with Drosophila gave 



