No. 526] SHORTER ARTICLES AND DISCUSSIONS 631 



that fertility increases, or, in other words the barrier, more or 

 less perfect between hybridization of the species involved, is 

 gradually broken down. As Kerner 1 " has shown, indeed, many 

 hybrid plants are more fertile than the parent spe.-ies and this 

 is maintained and increased during future generations, so that 

 the hybrids have replaced the parent species. There is, then, 

 a basis for believing that a kind of acquired "congeniality" 

 obtains, whereby the conjugating cells become more compatible, 

 whatever this may ultimately involve. An interesting case was 

 described by Gordon 19 where Nicotiniana, Digitalis and other 

 hybrids are sterile inter se, but fertile with the parent forms. 

 In the cross between the dog and the wolf, sterility begins, not in 

 the earlier generations, but in later ones, according to the obser- 

 vations of Flourens, but as Darwin remarks, it is doubtful that 

 this is due to increasing sterility because of crossing, but rather 

 to confinement or, indeed, to inbreeding. 



The students of plant cytology were the first to examine crit- 

 ically the structures concerned in reproduction in hybrids. 

 Gartner long ago observed the shrivelled pollen grains, as we 

 have said before, but that was before the modern cytological 

 methods came into use. Jancic 20 has contributed widely to this 

 department of research. He observed that the number of pollen 

 grains decreased in certain hybrid plants and that in the several 

 species he examined, this numerical reduction occurred in ap- 

 proximate amounts to one fourth, one half or three fourths the 

 average number of grains. Moreover, cytological examination 

 of the anthers showed that many of the nuclei after reduction of 

 the tetrads were atrophied, and the conclusion is obvious that 

 herein lies the explanation of his numerical ratios mentioned 

 above ; for in some crosses, three fourths of the nuclei destined 

 for the pollen (the daughter and granddaughter cells of the pollen 

 mother-cell), or, in other words, three of the four tetrads, 

 aborted, leaving but one to become pollen (hence the one fourth), 

 while in other cases, two of the tetrads disappeared (two 

 Museum a'histoire naturelle, Paris. 



- Hurst, C. C, 1900, Journ. Boy. Hortic. Soc, Vol. 24, p. 124. 



"De Vries, H., 1903. "Die Mututioiistheorie," Vol. 2, p. 66. 



"Kerner, A. von, "Kbnnen aus Bastarden Arten werdenf " Oesterr. Bot. 

 Zeit., Bd. 21, 1871. 



"Gordon, D. A., 1862. Mem, dead. StOnisl, p. 228. 



"Jancic, A., " Untersuchungen des Pollens hybrider Pflanzen," Oesterr. 

 bot. Zeitschr., 1900, Nr. 1, 2, 3. 



