62 PLANT GROWTH SUBSTANCES 



embryo culture we should be able greatly to Increase the number of 

 wide hybrids between species, and by the use of colchicine we should be 

 able to transform many of them into pure-breeding double diploids or 

 new species. 



Having found a method for doubling the chromosome number it 

 seemed desirable to find a method for halving the number. Such a 

 method would have considerable theoretical and economic importance. 

 A plant breeder, for example, may wish to combine in a single variety 

 of wheat the resistance to different rusts and smuts, high yield, good 

 milling, and other desirable qualities by crossing two varieties together. 

 The resulting hybrid will be highly heterozygous and it may take several 

 generations and much labor before he is able to secure a pure-breeding 

 type with the combination of characters he desires. If he could only 

 induce the reduced egg cells to develop without fertilization he could get 

 homozygous individuals in two jumps since the m plants from unfertil- 

 ized eggs can be readily induced to double their chromosome number — 

 at least such is the case in Datura. We confess we have not yet succeeded 

 in our attempts to induce unfertilized eggs to develop but we beheve 

 the problem is soluble since the plant does it as shown by the fact that 

 we have found over 200 spontaneous m plants in our cultures of Datura 

 stramonium. A related question is what are the factors which determine 

 whether a cell shall divide mitotically retaining the 2« condition in the 

 daughter cells or divide meiotically thus halving the chromosome num- 

 ber. Perhaps work now under way in the University of Wisconsin will 

 help us to the answer. Through the use of auxins one has a measure of 

 control over the production of roots. It would be of convenience to the 

 plant breeder if he were able similarly to induce shoot production. 

 (This sentence was written before I attended the meeting of the Growth 

 Societv in New London where I learned of the work of Skoog in con- 

 trolling bud and root formation through interaction of adenine and 

 auxin.) The plant breeder's next request would probably be for a 

 method of inducing flower formation or usable reduction divisions 

 wherever and whenever he wanted them. 



We now return to the question of our introduction: Why can't one 

 cross corn and beans and get succotash? This naturally leads us to 

 consideration of the barriers to crossability between species. We need 

 not mention at this time the various ways in which pollination is hindered 

 or prevented such as geographical separation, or flowering at different 



