304 Uuiversitij of Calif ODiia PubVications in Botany [Vol. 5 



first, very rapid early growth relative to their size and, second, an 

 ability to live over the winter season for a limited number of years. 

 Their rate of early growth brings them to maturity at approximately 

 the same date as their respective Tahacum parents, although they are 

 always somewhat larger than and in some cases twice as large as this 

 parent. The ability of the F^ hybrids to flower a second and even a 

 third season on their own roots has previously been taken to represent 

 a tendency inherited directly from their sylvestris parent. It now 

 seems probable, however, that this seeming perennial habit should 

 be looked upon simply as a manifestation or resultant of the greatly 

 increased vegetative vigor which they exhibit when compared with 

 their parents. This is in part suggested by the fact that the hybrids 

 do not die down and come up again from the base of the stem or 

 from the roots, but the less woody portions of the laterals and main 

 axis simply die back during the winter and masses of new laterals 

 from dormant or adventitious buds clothe the framework of the old 

 plant during the following growing season. 



The partial sterility of these Fj species hybrids should also be 

 briefly mentioned here (cf. Goodspeed and Ayres, 1916). Their flowers 

 produce only a small proportion of pollen of normal appearance and 

 it has not been possible to cause this pollen to germinate in its own 

 stigmatic fluid, in that of its parents, or in artificial germinating fluids. 

 In other words, the pollen of normal appearance produced by these 

 hybrids seems to be as strictly functionless as the evidently impotent, 

 shrunken grains which make up the greater portion of the contents 

 of the anthers. On the other hand, a few good ovules are produced 

 in these same flowers and viable seed is matured when they are open- 

 pollinated or crossed back with the pollen of the corresponding parent. 

 However, as has been indicated, apparently no seed can be produced 

 as a result of self-pollination, and indeed selfed F^ flowers fall a short 

 time after anthesis, due to a lack of successful pollination. In con- 

 cluding these general remarks it might be noted that reciprocal crosses 

 in this series of hybrids have always given results identical in every 

 respect. 



F,H36 — N. (Tabacum) angustifolia X N. sylvestris 



This hybrid is figured in plates 38, 43, and 46. Plate 38 exhibits 

 the hybrid placed between the two parents. Tlie plant of sylvestris 

 on the left is not a representative individual even so far as general 

 habit is concerned and was taken up from the garden during tho tbii'd 



