[Chap. XXXIX MUTATIONS 477 



have occurred in the meristem of the seedling, and has been perpetuated 

 in all branches from that part of the meristem. 



We may consider a case in which the division of the chromosomes was 

 not followed by further cell division and wall formation, resulting in a 

 cell with the tetraploid (4n) number of chromosomes. This aberration 

 is known to occur under natural conditions. The frequency of its occur- 

 rence has been increased experimentallv by exposing certain plants to 

 high or low temperature, by wounding them, or by treating them with 

 various chemicals, especially anesthetics and alkaloids. Examples are 

 the increased frequency of occurrence of 4n number of chromosomes in 

 the first cells formed from the zygotes of corn, wheat, barley, and sweet 

 clover when the flowers have been exposed to 40° -45° C. for a half hour 

 while the zygotes are dividing. Similar results were obtained with low 

 temperatures in Jimson weeds, and in some of the adventitious branches 

 that developed from wound surfaces in tomato stems. A weak solution 

 of chloral hydrate was one of the first chemical agents used to increase 

 the rate of chromosome doubling. In cells treated with this reagent 

 some or all of the chromosomes divided a second time while they were 

 migrating toward the poles of the spindle in the dividing cells. 



All these external factors are known to alter the polarity of cells and 

 the viscosity, permeabilitv, surface tension, and streaming of protoplasm. 

 It is probably through such indirect means that they influence the regu- 

 lar behavior of chromosomes. Spindle formation and the migration of 

 chromosomes are certainlv influenced by all of these internal conditions 

 in the protoplasm, except permeability. 



Recently it was discovered that an alkaloid, colchicine, from the 

 autumn crocus is very effective in preventing spindle formation and 

 subsequent cell wall formation in dividing cells, thus stopping the 

 process with the splitting of the chromosomes. These treated cells con- 

 tain the tetraploid (4n) number of chromosomes. The plant, or some 

 part of it, may be soaked in a very dilute solution of colchicine, or the 

 solution may be sprayed upon the plant. How far botanists may go in 

 increasing the number of chromosomes in plant cells by this method will 

 have to be decided by future experiments. 



Even before the discovery of colchicine and other means of increasing 

 the number of chromosomes in a cell, it was known that tetraploid ( 4n ) 

 cells would survive and grow, and that whole plants bearing flowers 

 could be obtained from them as described above. They had been found 

 in the wild state. In such plants the gametes contain the diploid (2n) 



