June 2, 1922] 



SCIENCE 



599 



ber of pollen tubes in various portions of the 

 style at any given time after pollination. 



Though a lateral displacement of the pollen 

 tubes results from the flattening of these 

 strands of conducting tissue, every pollen tube 

 is practically in place with reference to its 

 distance from the stigma or ovary. By means 

 of a microscope equipped with a mechanical 

 stage it was found possible to count their 

 number and measure their distance from the 

 end of the stigma, down as far as they had 

 penetrated, from which their curves of dis- 

 tribution could be plotted and studied. 



In the adjoining diagram the pollen tube 

 distribution curves were made by superposing 

 the pollen tube counts of a dozen or more 

 styles whose foremost pollen tubes had pene- 

 trated to about 42 mm. The counts were made 

 for 2mm. intervals and this represents their 

 distribution about fourteen hours after pol- 

 lination under fairly uniform temperature 

 conditions — approximatelj' 20°C. The bi- 

 modal curve is for Globes selfed and repre- 

 sents a total of 8,365 pollen grains applied to 

 18 stigmas under similar conditions, while the 

 curve of distribution for selfed normal plants 

 is shown superposed on this and represents 

 4,691 pollen grains applied to 11 different 

 stigmas. In the normals the germination was 

 95.6 per cent, while the Globe pollen selfed 

 gave a germination of 94.9 per cent. The 

 curves are much more jagged when the pollen 

 tube populations from individual styles are 

 plotted but those from Globes are quite as 

 characteristically bi-modal. 



The explanation offered is that though the 

 Globe pollen selfed germinates about as well 

 as the normal pollen selfed, there are slower 

 growing pollen tubes among the rapidly grow- 

 ing ones and soon this population of game- 

 tophytes becomes resolved into two groups 

 which grow at slightly different rates. This 

 bi-modal character increases with time, and 

 ' the slowest poUen tubes may fail to fertilize 

 because they fail to enter the ovary before 

 abscission of the style, or they may fail only 

 because the ovules were already fertilized by 

 the more rapid pollen tubes. Since the Globe 

 character is only slightly transmitted through 

 the pollen, we infer that the pollen tubes with 



(w+1) chromosomes are the slow ones, while 

 the tubes with n chromosomes are those in the 

 lead. 



While this study is very largely still in its 

 preliminary stages, it seems to show that we 

 have in Datura a selection between gameto- 

 phytes, one of the special forms of Develop- 

 mental Selection described by one of us (4), 

 thus proving that this form of selection is sub- 

 ject to experimental study. The result of our 

 preliminary study also shows that the Globe, 

 as well as the other (2n-\-l) mutants of Da- 

 tura, illustrates a condition in which the mu- 

 tations tend to disappear because they are not 

 favored by the processes of Developmental 

 Selection. 



Literature Cited 



1. Blakeslee, A. ¥., 1922 ; Variations in Da- 

 tura due to changes in chromosome number. 

 American Naturalist, 56: 16-31, Jan.-Feb. 



2. , 1921; The Globe Mutant in the 



Jimson weed (Datura Stramonium). Genetics, 

 6: 241-264, May. 



, 3. Blakeslee, A. F., John Belling and M. E. 

 Farnham, 1920; Chromosomal duplication and 

 Mendelian phenomena in Datura mutants. Science, 

 N. S., 52: 388-390, Oct. 22. 



4. Buehholz, J. T., 1922; Developmental Se- 

 lection in vascular plants. Bot. Gas., 73 : 249-286, 

 Apr. 



John T. Buchholz, 

 Albert F. BijAkeslee 

 Station fob Experimental Evolution, 

 Cold Spring Harbor, L. I. 



THE MATHEMATICAL ASSOCIA- 

 TION OF AMERICA 



The sixth annual meeting of the Mathemat- 

 ical Association of America was held at the 

 University of Toronto on Thursday and Fri- 

 day, December 29 and 30, 1921. One hundred 

 and ten were in attendance at the sessions of 

 the association, 88 of these being members of 

 the association. The following papers were 

 read at the meeting aside from the papers by 

 Professors Carmichael, Cui'tiss and Slaught 

 on the program of the joint sessions with the 

 American Mathematical Society, and Section 

 A of the American Association : 



