DEPARTMENT OF GENETICS. 99 



Gametophytic Selection in Datura. 



The investigations of Dr. John T. Buchholz, who has been associated with 

 Dr. Blakeslee, have been carried on at Cold Spring Harbor during the sum- 

 mers of 1921 and 1922. In regard to the selection between male gameto- 

 phytes, he reports as follows: 



"In the Globe mutant the pollen-tubes with (n+1) chromosomes are slower 

 in their growth than the pollen-tubes with (n) chromosomes. This is shown 

 by the fact that the pollen-tubes are grouped in a bimodal curve of distri- 

 bution when Globe pollen is applied to the stigma, while normal pollen under 

 comparable conditions forms essentially a unimodal curve of distribution. 

 The fact that the pollen transmits the Globe character-complex to only a 

 slight degree when pollen from Globe plants is placed on normal plants is 

 therefore explained on the basis of differential pollen-tube growth." 



Dr. Buchholz is also working on a selection of ovules within the ovary and 

 seed capsule — "interovular selection." 



The Cytology of Datura Mutants. 



As his share of the joint cooperative work on Daturas under Dr. Blakeslee, 

 Dr. John Belling has paid particular attention to the tetraploids. He 

 reports as follows : 



"Normal tetraploids. — In the late prophase and early first metaphase in the 

 pollen mother-cells, the 48 chromosomes of a normal tetraploid Datura are 

 usually connected in groups of four (quadrivalents) . At the anaphase these 

 separate 2 and 2, or 1 and 3 (non-disjunction). Examination of the distri- 

 bution at the second metaphases shows that a division into 24 and 24 has 

 occurred in from three-quarters to two-thirds of the pollen-mother-cells, while 

 the others mostly show 23 and 25, and rarely 22 and 26, etc. 



"Normal tetraploids with 12 sets of 4 homologous chromosomes each, 

 should, if the chromosomes in the megaspores are distributed as in the pollen- 

 mother-cells, and if any of the 23 or 25 chromosome pollen functions, give 

 some progeny with 49 and some with 47 chromosomes (and more rarely 

 plants with 46 or with 50 chromosomes). This is the case." 



"Abnormal tetraploids. — The occurrence of double opposed non-disjunction 

 would lead to the production of gametes with 24 chromosomes, having only 

 1 of one set and 3 of another. Crossed by a normal 24-chromosome gamete, 

 the resulting plant would have 48 chromosomes, including 3 of one set and 5 of 

 another. Such plants might show over 50 per cent of apparent non-dis- 

 junction, and might give up to a quarter of 47-chromosome or of 49-chromo- 

 some plants among their progeny, and up to one-sixteenth of 46-chromosome 

 or 50-chromosome plants. Two plants with this large amount of non-dis- 

 junction have been specially studied. 



" Tetrads of normal tetraploids. — Normal tetraploids have rarely shown cases 

 of non-reduction (leading to the formation of 48-chromosome pollen-grains). 

 Two pairs of giant cells have, however, been met with among nearly 3,000 

 tetrads. Such pollen-grains, if functional, would produce hexaploid (Qn) 

 plants. 



"Progeny of tetraploid pollinated by normal. — This difficult cross has resulted 

 in 27 plants, of which 25 have had sufficient chromosome sets counted, and 2 

 are still somewhat uncertain. 14 were diploid and 2 had 13 + 12 chromosomes. 

 The origin of these is uncertain. Six plants of this cross were triploid; one 

 had 35 and another 37 chromosomes. Judging from this evidence as to the 



