860 



SCIENCE 



[N. S. Vol XLIII. No. 1120 



County, where all possibility of crossing is ex- 

 cluded, there was found a most unexpected 

 amount of shrivelled pollen grains. One 

 flower was examined from each plant. The 

 flowers have three anthers and in some cases 

 anthers from the same flower yielded differ- 

 ent percentages of bad grains. Yet the 

 anthers, and the plants as a whole, were all 

 entirely normal in appearance. The amounts 

 of bad pollen are shown in the following table : 



Pollen of Scoliopus Bigelovii 



Bad Grains Individual Antliera 



Plant Per Cent. Per Cent. 



No. 1 31.9 45.4 



25.8 

 33.2 



No. 2 20.6 — 



No. 3 18.5 6.4 



25.6 

 6.5 



No. 4 10 10.7 



9.04 



No. 5 9.9 11.1 



7.8 

 10.4 



No. 6 3.75 — 



No. 7 3.25 — 



Thus, in the absence of crossing, these plants 

 in their normal habitat produce from 3 per 

 cent, to 32 per cent, of bad pollen, and in indi- 

 vidual anthers the observed amount exceeded 

 45 per cent. This in itself is a sufficient refu- 

 tation of the hypothesis that bad pollen is nec- 

 essarily a sign of hybridity. It would be diffi- 

 cult to find a plant which is more suitable for 

 disproving this hypothesis than Scoliopus 

 Bigelovii. It furnishes all the conditions that 

 the most captious critic could desire, inclu- 

 ding relative unifoiinity and the absence of a 

 related species with which it might cross. 

 Yet, with two exceptions, it shows a higher 

 percentage of bad grains than any other plant 

 examined. 



Dirca occidenialis furnishes an even more 

 convincing proof that bad pollen may occur in 

 quantity in plants that are not hybrids. This 

 shrub belongs to an isolated genus of the 

 Thymeleacese, the only other species being 

 found in the eastern states. Pollen examined 



from three separate flowers on the same 

 branch yielded respectively 8.7 per cent., 20.8 

 per cent, and 46.6 per cent, of bad grains. 

 Many of the pollen grains are also conspicu- 

 ously undersized, so that the amount of non- 

 viable pollen in this plant apparently often far 

 exceeds 50 per cent. 



The pollen of two other species taken at 

 random has also been examined, with the fol- 

 lowing results. 



Ranunculus Californicus showed in one 

 case 21.7 per cent, bad pollen and in another 

 case 4.4 per cent. The pollen of Fritillaria 

 lanceolata var. florihunda appears to contain 

 regularly more than 50 per cent, of bad grains. 

 These are both variable species, and in this 

 case the possibility of crossing is not excluded. 

 They are included here so as to avoid the pub- 

 lication of selected results. 



It is certain, then, that bad pollen, even 

 when it occurs in large amount, is not neces- 

 sarily an indication of hybridization. Pollen 

 sterility is rather a physiological condition 

 which occurs in all degrees of intensity and 

 may be due to a variety of causes. Hybridi- 

 zation is of course one of these, but only one. 



Multiple causes apply in the same way to 

 the conditions of sterility in animals. The 

 mule is sterile because it is a hybrid whose 

 parents are not only very dissimilar but have 

 different chromosome numbers.^ On the other 

 hand, the various species of the genus Bos 

 apparently intercross freely without any sign 

 of sterility.' To cite one other case of sterility 

 of an entirely different character, Morgan* 

 showed that in certain generations of Phyl- 

 loserans in spermatogenesis, half the sperma- 



2 Wodsedalek, J. E., 1916, "Causes of Sterility 

 in the Mule," Biol. Bull., 30: 1-57, Pis. 9. 



3 Similarly, Dorsey ("Pollen Development in the 

 Grape with Special Reference to Sterility," Univ. 

 of Minnesota Agric. Espt. Sta., Bull. 144, 1914) 

 concludes that in grapes hybridity is not necessarily 

 a cause of sterility, since both sterile and fertile 

 hybrids occur among cultivated varieties. 



* Morgan, T. H., 1909, "A Biological and Cyto- 

 logical Study of Sex Determination in Phylloxer- 

 ans and Aphids," Jour. Expt. Zool., 7: 239-352, 

 PI. 1, Figs. 23. 



