492 
Journal of Agricultural Research 
Vol. XXVII, No. 7 
Isolation of these Pima families in which the spot is absent or nearly 
so has afforded opportunity for further study of the inheritance of the 
character, this time within a single variety. Crosses were made in 1920 
between individuals of an inbred Pima family showing normal develop¬ 
ment of the spot and individuals of the families in which it is very weakly 
developed. The descendants of these crosses have been studied in the 
first, second, and third generations and the inheritance of the character 
in the intra-varietal crosses proves to be similar to that which occurred 
in the interspecific hybrid Holdon (upland) X Pima (Egyptian). 
The heredity of the cotton plant has been studied rather extensively, 
but definite Mendelian segregation has been demonstrated in relatively 
few characters. For this reason it is advisable to give in some detail the 
facts as to the inheritance of petal spot, although, from the standpoint 
of genetics, the case presents no exceptional features. The subject has, 
however, a practical bearing, for the possession by an agriculturally 
valuable strain of Pima cotton of such a character as “ spotless ” petal 
would protect the purity of the seed by making it easy to detect the 
offspring of accidental cross-pollinations with other strains. Proof that 
this character behaves as a simple Mendelian recessive warrants the 
expectation that it can be transferred without great difficulty to any 
desirable strain of Pima cotton. 
METHOD OF DETERMINING THE CHARACTER 
No satisfactory method for the accurate quantitative measurement of 
the petal spot suggested itself, so a system of grading was employed 
for the classification of the flowers, as in the earlier study of the upland- 
Egyptian hybrid. Complete absence of the spot was indicated by 
grade o and its highest development by grade 9. Grades o, 3, 6, and 9 
are represented in Plate 1. The endeavor was to have the grades indicate 
the total quantity of red pigment present, rather than merely the size 
of the area or the intensity of the color. Thus a flower having a larger 
but lighter colored spot might be graded the same as a flower in which 
the spot was smaller but more deeply colored. It is not assumed that 
the grading was done with perfect consistency, but it is believed that 
the deviation from the standard in no case exceeded one full grade and 
that the results obtained by this method are entirely satisfactory for 
the purpose of this analysis. For statistical treatment of the data, the 
method of grading has the advantage of making possible, in a given 
length of time, the classification of much greater numbers of flowers 
than could be handled if it were attempted to determine quantitatively 
the amount of pigment present. 
In grading the flowers those representing the two extremes of the 
scale gave the most trouble. Flowers which appeared to be completely 
spotless when viewed with the unaided eye showed in some cases, when 
examined with a hand lens, very faint traces of color in the region of the 
spot, the red pigment being confined to scattered very small groups of 
cells. The practice adopted was to grade as o all flowers which showed 
no trace of red when examined in a good light without magnification. 
Experience indicated that the scale used should have been lengthened 
somewhat at the upper end, and that if the distinctions had been drawn 
as finely in this region as elsewhere it would have been necessary to 
recognize one or possibly two grades above grade 9. 
