HEREDITY AND VARIATION IN PLANTS 147 



high humidity to prevent injury. We are unable to explain 

 . the way the high temperature influences cell division. 



Perhaps the most striking and the most successful chemi- 

 cal treatment for inducing polyploidy is the use of colchicine, 

 a poisonous substance from the bulbs of fall-blooming crocus. 

 This chemical had been known for some time to check cell 

 division of animal cells, but not until 1937 was its potent 

 effect on plant cells demonstrated. Dr. Nebel and Dr. 

 Ruttle, his wife, at the New York Experiment Station at 

 Geneva and Dr. Blakeslee of the Cold Springs Harbor labora- 

 tory of the Carnegie Institute, working independently dis- 

 covered that colchicine caused a doubling of the chromosome 

 number. The explanation of these results does not go very 

 far but in some way the chemical prevents the formation of 

 the cell wall that normally separates the chromosomes after 

 they have divided in cell division. In other words, the divi- 

 sion is incomplete. This may be repeated several times; 

 Levan in 1938 reports cells of the root-tip of onion with 

 "probably" about 500 chromosomes. As the chromosomes 

 increase in number, the size of the cells increases and at the 

 same time the organism and all of its parts increase in size. 

 Apparently, however, cell division becomes slower and more 

 difficult as the chromosome number increases, causing the 

 death of the organism when too large. At present the suc- 

 cessful use of colchicine appears to depend on a treatment 

 that is strong enough to cause the chromosome number to 

 double once in a large proportion of the cells of the treated 

 portion. Stronger treatment is lethal. A treated bud that 

 forms a branch, or a seed forming a plant with the increased 

 number, can be used for vegetative reproduction or in seed 

 formation. 



Methods of using colchicine have been described by Drs. 

 Blakeslee and Avery of the Cold Spring Harbor Laboratory, 

 some of which are illustrated by Plate VIII. Seed treatment 

 is probably the easiest and most effective. They have in- 



