i^--l Setckell-Gaodspeed-Clausen: Nicotiana Tabacum 517 



of pairs of chromosomes may account for the ease with which recom- 

 bination pure lines were established. Even with a large number of 

 factor differences, such as evidently distinguish these Tabacum varie-' 

 ties, the chances are slight with so many pairs of chromosomes that 

 linkage will enter in as a factor to cause the continued preservation 

 of a heterozygous condition as a consequence of selection for a certain 

 set of characters. 



It remains to consider those portions of the Nicotiana literature 

 which deal specifically with the Mendelian inheritance of the charac- 

 ters which we have investigated, and to harmonize our results with 

 those which have been reported previously. Unfortunately there have 

 not been many investigations in Tabacum which have been prosecuted 

 far enough to arrive at a definite factor analysis of the differences 

 under consideration. The investigations of Miss Howard (1913), 

 " promise of the continuation of which has not thus far been fulfilled, 

 in general confirm those which we have presented in this paper. On 

 the strictly analytic side, however, Miss Howard did not carry her 

 work very far. This doubtless was due to the difficulty of making a 

 factor analysis of the characters which she selected for study, viz., 

 (1) time of flowering, (2) height of stem, (3) arrangement of the 

 leaves on the stem, (4) length of the decurrent portion of the lamina, 

 (5) venation of the leaf, (6) leaf shape, and (7) undulation of the 

 surface and margin of the leaf. For most of these characters she 

 demonstrates, by the presentation of numerical data in some cases 

 as far as F^, the probability of the character differences in ques- 

 tion depending upon multiple factor differences. In the case of height 

 certain of her cultures strongly suggest the existence of a pair of 

 allelomorphs, which has a relatively great effect, for in some of her 

 cultures there are definite discontinuous height differences. For the 

 inheritance of length of the decurrent portion of the lamina Miss 

 Howard postulates the existence of at least three or four distinct pairs 

 of factors. As respects leaf base, she records the synthesis of petiolate 

 types from sessile parents, observing in two cases a simple 1 :2 :1 segre- 

 gation into petiolate : intermediate : sessile. As respects corolla color, 

 she records one Fj population from pink x very pale pink fading into 

 white which consisted of 72 pinks of various shades to 45 whites, but 

 some of the palest pinks were indistinguishable from Avhite. She found 

 evidence of grouping among the pinks, and postulates the existence 

 of two factor differences to account for it. The investigations which 

 we have reported do not throw light upon the factor constitution of 

 the very pale pink varieties with which Miss Howard worked. Our 



