8 
BULLETIN 1164, U. S. DEPARTMENT OF AGRICULTURE. 
A high lobe index indicates a shallow- 
Holdon gave a higher mean lobe index than Pima. 
as a percentage of the length of the leaf, 
lobed leaf and vice versa/ 
Leaf vein angle (fig. l,f a g) is the width, in degrees, of the angle subtended by 
the principal lateral veins. Of the parental populations, Pima gave the higher mean 
for this character. 
Leaf extension index is the length of the basal prolongation of the blade, below a 
line bisecting the base of the mid vein at a right angle to the latter (fig. 1, h k), 
expressed as a percentage of the length of the leaf. A high extension index indicates 
a relatively great prolongation of the base The mean leaf extension index was 
higher in Holdon than in Pima. 
Leaf callus color was graded in the field with the aid of types consisting of a set 
of leaf bases representing the range from grade 1 (callus pale brownish green) to 
grade 9 (callus liver colored). The grading was done on newly unfolded leaves in 
which the color was freshest. A'ariations in the size of the colored area, in hairiness, 
and in the general color of the leaf blade made it difficult to determine the true grade 
of some of the F 2 plants. Of the parental populations, Holdon gave the higher mean 
grade for callus color. 
Petiole hairiness was graded with the aid of types consisting of a set of petioles rep- 
resenting the range from grade 1 (glabrous or very nearly so) to grade 8 (extremely 
hairy). The petiole was 
found to be the most con- 
venient organ for accurate 
grading of the character of 
hairiness. Ingeneral, the 
hairiness of the petiole 
appeared to be closely cor- 
related with that of the 
lower surface of the leaf 
blade, although in some 
of the F 2 plants the leaf 
surface was decidedly 
hairier than the petiole 
and in one plant the con- 
verse was true. The de- 
gree of hairiness is modi- 
fied by the age of the leaf, 
young leaves being 
usually much hairier than 
mature ones. The grading 
was done on the petioles of 
approximately half-grown 
leaves. A further com- 
plication was the diversity 
in F 2 in respect to the na- 
ture of the hairiness, which 
varied from short and felt- 
like to long and almost 
bristly; but as the number 
rather than the length or 
character of the hairs was taken as the criterion, little difficulty was experienced in 
fixing the grade number of most of the plants. Holdon gave a higher mean grade for 
petiole hairiness than Pima. 
Pedicel length, in millimeters, was that of the flower on which the other measured 
characters were determined. Pima normally has a pedicel much longer than 
Holdon. 
Bract length, in millimeters, was that of the longest of the three bracts in the 
involucre of the flower on which the other measured characters were determined. 
Pima gave a higher mean for bract length than Holdon. 
Bract connation index, determined on the same involucre, is the distance from the 
base of the involucre to the base of the sinus formed by the two most connate bract 
margins, measured in millimeters and expressed as a percentage of the length of the 
longest bract in the involucre. A high connation index indicates that the bracts 
are strongly connate. The mean index of bract connation was higher in Pima than 
in Holdon. 
Fig. 1.— Diagram of a cotton leaf, showing the location of the measure- 
ments: Length (a-b), width (c-d), sinus distance (a-c), vein angle 
(J-a-g), basal extension (h-k). 
9 Balls (>>, p. 1JS) used the same expression of depth of lobation. The writer in a recent publication 
(?6, p. 230, footnote 6 to Table 1) used the converse (leaf length as a percentage of sinus distance) as the 
lobe in^ex. This expression has the advantage that a high lobe index indicates a deeply lobed leaf and 
vice versa. But as the correlations in the Holdon X Pima hybrids had been previously calculated on the 
other basis, it seemed advisable to retain the above expression in this bulletin. 
