338 
Journal of Agricultural Research voi. xxvii, no. 6 
The data given in Table V show in no case a significantly greater 
percentage of hybrids from seeds contained in the lower halves of the 
bolls than from seeds contained in the upper halves, and therefore 
afford no evidence that the rate of growth of the tubes of the unlike 
pollen is inferior to that of the like pollen when the two kinds of pollen 
are in direct competition. 
The pistils of the Upland varieties are much shorter than the Pima 
pistils, but the Upland pollen fertilized equally well the ovules in the 
lower and in the upper half of the Pima ovary. Hence there is no 
evidence of a correlation between the length of the pistil and the length 
attainable by the pollen tubes, such as has been observed in other plants. 1 * * * * * 7 
DISCUSSION 
Evidence has been presented by one of the writers that a large majority 
of the ovules normally are self-fertilized in both the Egyptian and the 
Upland type of cotton (j,p. 9,10). It was also shown that pronounced 
selective fertilization occurs in the Egyptian type, but no conclusion 
could be drawn as to the general importance of this phenomenon as a 
factor in the observed preponderance of self-fertilization, owing to lack 
of conclusive evidence that selective fertilization takes place in the Up¬ 
land type also (7, p. 42-49 , 64). The results of experiments described 
in the present paper make it clear that selective fertilization occurs in 
both types and in about the same degree. Nearly 75 per cent of the 
ovules, on the average, are fertilized by pollen of the same type and 
variety when pollen of the other type is present on the stigmas simul¬ 
taneously and in approximately equal quantity. 
This evidence of pronounced selective fertilization should be considered 
in connection with the evidence given in another paper (7, p . 54, 55, 6j) 
that as a rule a majority of the pollen grains reaching the cotton stigmas 
have originated in the same flower, in other words that there is usually 
present on the stigmas an excess of self-pollen over foreign pollen. 8 
These findings afford a satisfactory explanation of the fact that self- 
fertilization predominates in cotton although the flower is so admirably 
adapted to cross-pollination (7, p. 12). 
Egyptian cotton, although apparently not referable to any one of 
the species of Gossypium recognized by taxonomists, is very closely 
related to G. barbadense L. and to G. peruvianum Cav. and exhibits 
differences from Upland cotton ( G.hirsutum L.) which are of specific if 
not subgeneric magnitude (< 8 } p. 4-6 , Plates /, III , V-XI). Never¬ 
theless, the results of experiments described in another paper (7, p. 
40-42) have shown that when Upland pollen alone was applied to the 
stigmas of Pima flowers the fertilization attained was at least equal to 
that resulting from the application of Pima pollen alone. It was found 
also that the reciprocal pollinations on Upland showed only a slight 
1 McClelland (q) ascertained that when two species of Vanilla, one of which had a relatively short and 
the other a long column, were cross-pollinated, pollen of the first type fertilized few or none of the ovules 
near the base of the ovary of the second type, while pollen of the second type (long column) fertilized the 
basal ovules of the first type (short column) even more readily than did its own pollen. On the other hand, 
Tokugawa found that when two species of Lilium, one having a long style and the other a short style, 
were reciprocally cross-pollinated, the pollen tubes of either species grew faster in the pistil of the same spe¬ 
cies than did the foreign pollen tubes ( io , p. 29, Table XU). 
8 The seeming discrepancy that under natural conditions more of the ovules appear to be self-fertilized 
in Egyptian than in Upland cotton, although selective fertilization occurs in both types in practically 
the same degree, is removed by the observations previously recorded (7, p. 34-36) that the Upland flowers 
usually receive foreign pollen in greater quantity than the Egyptian flowers. 
