106 CARNEGIE INSTITUTION OF WASHINGTON. 



ble number of double trisomic mutants with 2 sets each having an extra 

 chromosome. 



"This year we are finding chromxosomal mutants in considerable numbers 

 among our tetraploid pedigrees and before the season is over hope to identify 

 a large majority of the simple pentasomic tetraploids corresponding to the 

 simple trisomic diploids earlier discovered. Judging from appearances, we 

 doubtless have double pentasomic forms in our pedigrees, but their classifi- 

 cation must wait for cytological confirmation. 



"A single case of a deficiency in chromosome number associated with 

 semisterility of pollen has been found in a small trial pedigree from the cross 

 triploidX tetraploid. Probably a considerable number of simple and double 

 deficiencies maj^ be expected from the sowing of a larger number of seed of 

 this cross, which waits an opportunity of planting. 



"The interest in the daturas does not lie chiefly in an accumulation of 

 chromosomal types. The value of the work will lie, it is believed, rather in 

 a possible closer insight into the nature of the chromosomes and processes of 

 inheritance. The unbalanced mutants give us an opportunity of analyzing 

 the contribution of each chromosome to the final structure and physiology of 

 the plant. 



'The four different Globe mutants will best illustrate the effect of the 

 unbalanced condition. The simple tetrasomic Globe is a diploid, unbalanced 

 by 2 extra chromosomes in the Globe set. The 2 extra chromosomes are 

 more or less antagonistic to 1 1 sets with 2 members each, giving an unbalance 

 of 2 to 22. In comparison with the simple trisomic Globes where the un- 

 balance is 1 to 22, all the Globe characters in the tetrasomic Globe are 

 increased in expression. The simple pentasomic Globe tetraploid has an 

 unbalance of only 1 to 44 and, as one would expect, the Globe characters are 

 less distinct than in the simple trisomic Globes with an unbalance of 1 to 22. 

 With this relation in mind, simple hexasomic Globes were sought for in 

 which the unbalance should be 2 to 44 or 1 to 22, the same unbalance found 

 in trisomic diploid Globes. They were identified in offspring of pentasomic 

 Globes and their classification confirmed by chromosomal counts. Approxi- 

 mately half the pollen grains of simple trisomic Globes have an extra chromo- 

 some and their gametophytes therefore are unbalanced by 1 to 11. In 

 consequence, very few male gametes are effective in fertilization, the tri- 

 somic character being transmitted to only about 2 per cent of the offspring. 

 In the gametophytes of pentasomic Globe tetraploids the unbalance is less 

 (1 to 22), so apparently we find that pentasomic Globes transmit the extra 

 chromosome through the pollen to a much higher percentage of their offspring 

 (about 15 per cent in this year's planting). 



"The unbalanced condition gives us an opportunity, never before realized, 

 of analyzing the influence of individual chromosomes without waiting for the 

 appearance of gene mutations. Heretofore, the number of factors determined 

 in the chromosomes has been dependent upon the number of mutated genes 

 available for crossing with the normal type. In the jimsons, however, we 

 may study the sum total of all the factors in individual chromosomes by the 

 unbalancing effect upon the structure and physiology of the plant when a 

 single specific chromosomal set has 1 or 2 extra chromosomes." 



While the advance of knowledge of influence of single entire chro- 

 mosomes on the developing plant-body has been great during the past 

 12 months, yet the number of new problems in this field has increased 

 in the same ratio. The need for additional workers with this extra- 

 ordinarily valuable material is great. For the most part, the addi- 



