314 University of California Puhlications in Botany [Vol. 5 



of the sylvestris set of factors are apparently responsible for the 

 increased size and vigor of the hybrids and for slight proportional 

 variations which may appear in the degree of expression of some of 

 the Tahacum characters. 



The predominating influence of Tahacum in determining the char- 

 acters of Tahacum-sylvestris hybrids has also been observed by other 

 investigators. Thus Bellair (1911) has reported that when sylvestris 

 is crossed with Tahacum the F^ hybrid reproduces the characters of 

 the Tahacum parent, although the hybrids were larger and more 

 floriferous than that parent and were almost completely sterile. The 

 colored plate presented by Baur (1914) illustrates this same condition, 

 but not in such striking fashion as we have observed it. Unfortunately 

 this colored plate is not accompanied by any statement as to the type 

 of Tahacum used in the hybrid, consequently we do not know whether 

 the Tahacum variety figured is the actual parent of the F, hybrid 

 of Tahacum X sylvestris illustrated in the plate. The characters of 

 the sylvestris flower, however, are very faithfully portrayed. East 

 and Hayes (1912) have also reported results of the hybridization of 

 sylvestris and Tahacum, but almost wholly from the standpoint of 

 the increased vegetative vigor of such hybrids. Plate VI of this 

 article would appear, however, to bear out the contentions which have 

 been advanced as to the directive action of the Tahacum reaction 

 system. On the other hand. Brown (1912), who has made a rather 

 complete study of a hybrid between sylvestris and a variety of 

 Tahacum which he calls "Havana," has arrived at a different result 

 and states as a general conclusion that neither parent exercises a uni- 

 form influence over any particular character of the hybrid or over 

 the sum total of characters. Since this study was based upon a con- 

 sideration of hybrids of only one Tahacum variety with sylvestris, 

 it is not difficult to see how the striking correspondence between 

 varieties of Tahacum and their F^ hybrids with sylvestris could have 

 escaped him. Although he finds that the hybrids in the main resemble 

 the Tahacum parent, he states that in general habit and form of 

 inflorescence they resemble sylvestris rather closely. These statements 

 are not, however, very well borne out by the figures he presents, and 

 such differences as may be noted from the Tahacum variety may 

 be ascribed to other causes such as we have pointed out above. 



On the basis of his study of the external characters of the parents 

 and of the hybrid between them, Brown assigns to the Tahacum par- 

 ents an influence of approximately two-thirds and to the sylvestris 



