5** 
Journal of Agricultural Research 
Vol. XXVII, No. T 
The question whether modifying factors are involved, in addition to the 
major factor which determines presence of the spot as contrasted with its 
complete or nearly complete absence, was not answered decisively. There 
were indications of a slight degree of segregation in F x and some of the 
dominant and recessive F s progenies showed apparently significant 
increase or diminution of the spot in comparison with the corresponding 
parental population. But it was found that environmental factors had 
contributed to these differences in F 3 by influencing the rate of develop¬ 
ment of the plants in the several populations. Furthermore, selection 
within the parental lines appears to have resulted in no perceptible 
modification of the spot. Yet, although the evidence at hand does not 
prove the existence of modifiers, it by no means justifies the conclusion 
that such factors are not involved. 
The character petal spot shows considerable variation on the individual 
plant, even among flowers which open on the same day. The variation 
was not greater in the heterozygous than in the homozygous populations. 
The degree of expression of the petal spot was found to be correlated 
positively and significantly with size of the flower, in both spotted and 
spotless populations. There was, however, no evidence of linkage. 
The spot is affected by the environment, being responsive to soil 
variations and also to seasonal changes. Differences were noted both as 
between different seasons and as between different periods of the same 
season. 
The spotless families are typical Pima in all other characters, except 
that they have given consistently a higher proportion of 4-lock bolls than 
normally spotted Pima populations grown under comparable conditions 
at the Sacaton Station. 
There is an entire absence of linkage between spotless petal and a 
relatively high mean number of boll-locks and the spotless condition has 
been observed only in the two families described in this paper. It is 
probable, therefore, that the progenitors of these families, although dis¬ 
covered in a population grown from ‘hulk 0 seed, were nearly related if 
not sister plants. 
Such a character as spotless petal should be useful as the “hall-mark ” 
of an agriculturally and commercially valuable stock of Pima cotton. 
The fact that spotless is recessive would make it easy, in roguing seed- 
increase fields of a strain possessing this character, to recognize the first 
generation hybrids resulting from accidental cross-pollination with normal 
Pima, since they would have the spot well developed. 
The recessive nature of spotless should make it readily transferable by 
hybridization to any desirable strain of this variety. No unfavorable 
results from such combinations need be anticipated, for the existing spot¬ 
less families are not inferior to the average of the variety in fruitfulness 
and in the properties of the fiber. 
