1928] H oiling sh ead : Chromosomal Chimeras in Crcpis 347 



most of the cells of the "crown g'alls" on sugar beets which can be 

 induced by inoculation with Bacterium tuniefaciens have the tetra- 

 ploid chromosome number. 



Nawaschin did not venture any suggestion as to causal agencies in 

 connection with his tetraploid sector in Crepis Dioscoridis. Lesley 

 believed that it was unlikely that cold played any part as a causal 

 factor in tomato chimeras, as only the roots seemed to be affected. It 

 has been suggested by Mr. C. W. Haney that watering greenhouse 

 plants ^vitll cold water would provide the necessary conditions if sud- 

 den lowering of temperature has anything to do with the production 

 of tetraploid root cells. The plants described here were entirely 

 normal as far as could be observed and tetraploidy could not be 

 ascribed to any special factor in the environment. 



Winkler had suggested that certain tissues may regularly become 

 polyploid and Breslawetz (1926) has reported tetraploidy as the 

 universal condition in the dermatogen of the root tips of Cannabis 

 sativa.'- De Litardiere (1923) found tetraploid and octoploid cells 

 in the dermatogen of Spinacia oleraeea. In both these cases it would 

 seem that the transforming of diploid into tetraploid cells must have 

 occurred many times in the same root. 



The possibility of fragmentation giving rise to these increased 

 numbers is easily excluded in most cases. The two most favorably 

 received theories to account for doubling are : ( 1 ) the fusion of nuclei 

 from two cells; (2) the division of the chromosome complex without 

 cytoplasmic division. Breslawetz has brought forward evidence that 

 nuclear fusion gave rise to the tetraploid cells which made up the 

 dermatogen of the roots in Cannahis sativa. As no diploid complexes 

 were to be seen in that region of the root one would conclude that 

 fusion of diploid to form tetraploid nuclei had taken place before any 

 normal diploid divisions occurred, or at least at an early stage in the 

 development of the root. If so, one wonders why evidences of nuclear 

 fusion were still to be found in well developed roots. On the other 

 hand, the paired condition of the chromosomes in some of the tetra- 

 ploid cells in Spinacia oleraeea convinced de Litardiere that these 

 cells had just completed a chromosome division without separation of 

 the resulting daughter chromosomes. 



The occurrence of multinucleate cells in a root of the Crepis 

 Bureniana plant which was partly tetraploid has been noted above. 

 The significance of this phenomenon in the origin of the tetraploid 



2 De Litardiere (1924) found rare diploid cells in the periblem and in one 

 case a tetraploid cell in the plerome of roots of this species. 



