52 
GENETICS: SETCHELL ET AL. 
Proc. N. A. S. 
certain forms as centers of distribution. Twenty F 2 selections with re- 
spect to the character of the leaf base were made and their progenies were 
studied in F 3 and subsequent generations. The twenty selections fell into 
six general leaf types, the names and descriptions of which follow : 
1. Stenophylla, long-petioled like angustifolia, and with a minimum 
wing development along the petiole. 
2. Latifolia, short but distinctly petioled like the Fi, and with a variable 
wing development. 
3. Lanceolata, broad-based sessile leaves with long lanceolate blades. 
4. LoriifoMa, sessile leaves with very narrow strap-like blades. 
5. Auriculata, leaves with the blades sharply constricted at the base, 
nearly if not quite to the mid-rib, and typically with clasping auricles. 
In some auriculata forms the auricles may be absent, in which case the 
leaf may appear to have a short distinct petiole. 
6. Sessilifolia, sessile leaves with a broad base and a broad clasping 
insertion. 
Of the above six types of leaves, three, lanceolata, loriifolia, and sessili- 
folia, differ merely in width of blade. Genetically they appear to be 
identical in their leaf-base factors. For the purposes of this discussion, 
therefore, it is only necessary to distinguish four primarily leaf -base types, 
stenophylla, latifolia, sessilifolia, and auriculata. 
As we have stated, F 2 gave no decisive evidence either as to the number 
of factors involved in the leaf -base contrast or as to the interactions among 
them. In order to obtain suitable analytical material we were obliged 
to study populations of F 3 and subsequent generations. In such popula- 
tions a notable simplification was observed in the number of segregation 
products, and the distribution into classes was more distinct. In the course 
of the investigations, also, constant derivatives of the various leaf-base 
types mentioned above were established, and further information as to 
the genetic relation of the different types was obtained by intercrossing 
these lines and by crossing them with the original parents. Theoretically 
such lines should differ less from the parents and from one another than 
the original parents did from each other, and practically this was found to 
be the case. The behavior in skeleton of selections of various leaf -base 
types is given below. 
Stenophylla selections were observed to segregate in a variety of ways. 
Two populations bred true for stenophylla; three segregated into 3 steno- 
phylla: 1 sessilifolia; and one gave 3 stenophylla: 1 auriculata. A latifolia 
derivative crossed with angustifolia gave F\ stenophylla, and F 2 3 steno- 
phylla: 1 latifolia. 
Latifolia selections proved rather perplexing. In some cases latifolia 
is evidently a complex hybrid expression, as is evidenced by the fact that 
the Fi cross between the original parents was typically latifolia in appear- 
ance. One selection repeated the complex F 2 segregation in F 3 ; two bred 
