204 



The Journal of Heredity 



tubes from cross-pollinations were taken 

 after they had grown down the style; 

 they averajijcd .OLS mm. Thus the 

 width of the i)ollen tubes in a cross- 

 pollination was nearly constant, beinj^ 

 slightly larger near the iJollen grain. 

 The largest diameter of a pollen tube in 

 a self-jjollination was .028 mm., while 

 the smallest was .016 mm. The largest 

 diameter of a pollen tube in a cross- 

 i:»ollination when the measurement was 

 taken close to the pollen grain was .024 

 mm., while the smallest was .012 mm. 

 When the measurements of the i)ollen 

 tubes from the cross-] )ollinations were 



POLLE.N GRAIN "HI DDINC;" 



When the iKjllen grain falls on the stigma 

 of a female blossom, moisture and 

 ehemieal reactions make it germinate. 

 Part of the contents push their wax' 

 through a pore in the wall, as shown 

 above in a jihotomicrograph of sweet- 

 pea jjollen hv John Howard Paine. 

 (Fig. 1.) 



taken down the style, the diameters 

 were also .024 mm. for the largest and 

 .012 mm. for the smallest. Thus the 

 greater width of the jjollen tubes from a 

 self-pollination is not so great as to 

 cause any mechanical difficulty to its 

 growth down the stylar canal, since 

 the smallest tube from a self-pollination 

 was of the same size as the largest 

 one of a cross-jjollination which was 

 measured down the style. 



With alsike clover ( Trifolium hyhri- 

 dimi) fourteen different i)lants were 

 used. There were \A^1 cr()ss-])ollina- 



tions and 1,452 self-pollinations made 

 on these plants. They were covered 

 with a netting so that the ]jollinations 

 could not be w^orked u])on by insects. 

 A number of flowers under the screen 

 were left untrij^jjed and a number of 

 flowers on plants outside of the screen 

 were left to be ojien-pollinated. Th^ 

 following table shows the percentages of 

 flowers set in the different pollinations. 



Total Per 



No. of No. of cent of 



fjoicers flowers flowers 



pflUiu- setting setting 



a led seed seed 



Cross-polhnations. . 1,437 309 21.50 



Self-pollinations... 1,452 40 2.75 



■•■'lowers not trii)ped 3,394 1 0.02 



Open-pollinated... 3,480 2,442 70.17 



In making a self-pollination the 

 flower was trip])ed by a i)air of tweezers 

 and no care was taken to jjrevent 

 ])ollen from one flower in a head from 

 pollinating another flower of the same 

 liead. In making the eross-i)ollina- 

 tions, the flowers were not emasculated 

 and the pollen from another plant was 

 dusted on to the stigma, using four or 

 live flowers to jjollinate a head. 



A number of cross-]X)llinated styles 

 were examined in a similar manner as 

 with Tradescantia, and in all cases 

 there was a large niunber of pollen 

 tubes growing throughout the length 

 of the style. The styles of forty 

 self-pollinated flowers were examined. 

 There w^as good germination on all of 

 the stigmas, but on thirty-nine of the 

 styles examined no pollen tubes were 

 found to extend over 1 mm. down the 

 style. In one style of the forty exam- 

 ined, three pollen tubes extended the 

 whole length of the style. This prob- 

 ably accounts for the low percentage of 

 flowers set from the self-pollinations. 



With alfalfa (Medicago saliva) the 

 flowers from eight diflerent i)lants were 

 used. The self and cross-iJoUinations 

 were made in the same way as with alsike 

 clover. The following table gives the 

 number of flowers and the i)ercentagcs: 



Total 



No. of Per 



flmcers No. of cent of 



pollin- flowers flowers 



a ted set set 



Cross-iH.Uinatiuns.. 822 318 38.7 



Self-pollinations... 1,459 396 27.1 



