K. — BOTANY. 173 



It will be apparent from the cases cited that we cannot infer from 

 I lie genetic analysis of one type that the factorial relations involved 

 are the same for the corresponding character in another. That this 

 should be so in wholly unrelated plants is not perhaps surprising, but 

 we find it to be true also where the nature of the characteristic and 

 the relationship of the types might have led us to expect uniformity. 

 This is well seen in the case of a morphological feature distinctive of 

 the N.O. Graminete. The leaf is normally ligulate, but individuals are 

 occasionally met with in which the ligule is wanting. In these plants, 

 as a consequence, the leaf blade stands nearly erect instead of spread- 

 ing out horizontally. Nilsson-Ehle * discovered that in Oats there are 

 at least four and possibly five distinct factors determining ligule forma- 

 tion, all with equal potentialities in this direction. Hence, only when 

 the complete series is lacking is the ligule wanting. In mixed families 

 the proportion of ligulate to non-ligulate individuals depends upon the 

 number of these ligule-producing factors contained in the dominant 

 parent. Emerson' found, on the other hand, that jn Maize mixed 

 families showed constantly a 3 to 1 ratio, indicating the existence of 

 only one controlling factor. 



From time to time the objection has been raised that the Mendelian 

 type of inheritance is not exhibited in the case of specific characters. 

 That no such sharp line of distinction can be drawn between the 

 behaviour of varietal and specific features has been repeatedly demon- 

 strated. As a case in point and one of the earliest in which clear 

 proof of Mendehan segregation was obtained, we may instance Datura. 

 The two forms, D. Stramonium and D. Tatula, are ranked by all 

 systematists as distinct species. Among other specific differences is 

 the flower colour. The one form has purple flowers, the other pure 

 white. In the case of both species a variety inermis is known in which 

 the prickles characteristic of the fruit in the type are wanting. It has 

 been found that in whatever way the two pairs of opposite characters 

 are combined in a cross between the species, the Fo generation is mixed, 

 comprising the four possible combinations in the proportions which 

 we should expect in the case of two independently inherited pairs of 

 characters, when each pair of opposites shows the dominant-recessive 

 relation. Segregation is as sharp and clean in the specific character 

 flower colour as in the varietal character of the fruit. Among the 

 latest additions to the list of specific hybrids showing Mendelian inheri- 

 tance, those occurring in the genus Salix are of special interest, since 

 heretofore the data available had been interpreted as conflicting with 

 the Mendelian conception. The recent observations of Heribert- 

 Nilsson ' show that those characters which are regarded by systematists 

 as constituting the most distinctive marks of the species are referable 

 to an extremely simple factorial system, and that the factors mendelise 

 in the ordinary way. Furthermore, these specific-character factors 



^ KreuzunqaunttTsuchungen an Hafer und Weizen, Lund, 1909. 



•* Annual Eeport of the Agricultural Experiment Station of the University of 

 Nebraska, 1912. 



■' Experhnentelle. Studuv uber Vnriahilitnt, Spalf.iniff, Artbildunff und 

 Evolution in der Gattung Salix, 1918. 



