544 GENERAL REVIEW. 



keep an equal pace with each other, though this difference has 

 no influence on the typical form of the hybrids which arise 

 from the union. Still, the shorter or longer period necessary 

 for the transformation of one pure species into another by 

 hybrid fecundation seems to depend in some measure on this 

 difference. 



When two species, such as Nicotiana rustica and glutinosa, 

 do not admit of union with one another, or, as N. paniculata 

 and tabacum, only of imperfect union, the union may be ac- 

 complished sometimes by a third species which stands in close 

 elective affinity with either of the first. Thus Nicotiana rustico- 

 paniculata is completely fructified by the pollen of ghitinosa, as 

 also paniculate - rustica by that of tabacum. This affinity is 

 called by the author compensating or mediate affinity. The 

 pecularity in these unions is, that the consequent hybrids are 

 generally so like the type of the compensating species that 

 they can be considered merely as varieties of this latter which 

 furnished the fecundating pollen, and are for the most part 

 perfectly barren. 



Nierembergia. A small but elegant-habited genus of half- 

 hardy American annuals or perennials, easily propagated from 

 spring-sown seeds. N. gracilis, N. rivularis, and others, are 

 well known in gardens. 



N. gracilis picta is a hybrid and improvement upon N. 

 gracilis, being intermediate between that and N. frutescens, 

 having the green branching and profuse-flowering habit of the 

 former, with a stronger constitution ; the flowers of a blue tint 

 margined with white. 



Petunia (Petunias). A small ornamental genus of South 

 American plants, represented in our greenhouses by P. violacea, 

 a rather elegant-habited purple-flowered plant, and P. nyctagini- 

 flora (see * Bot. Mag.,' t. 2552), a white-flowered species having 

 large, oblong, opposite, viscid leaves. Seeds grow readily sown 

 in heat like Calceolarias, and good varieties may be perpetuated 

 by cuttings. Both plants hybridise very freely, and their off- 

 spring are characterised by a wonderful variability in habit 

 and colour.. M. Naudin (see 'Jour. Royal Hort. Soc.,' 

 1866, p. 6) remarks that these two species are perfectly well 

 defined, neither varying from seed, but intercrossing easily 

 and yielding hybrids as fertile as themselves. " In the first 

 generation all the hybrids are alike ; in the second (see Da- 

 tura}, they vary in the most remarkable degree, some revert- 

 ing to the white species, others to the purple, and a large 

 residue showing all the shades between the two. When these 

 varieties are fecundated artificially by each other, as is the 



