Self-sterility in Dewberries and Blackberries 
25 
Germination Test of Pollen. —Having discovered that self-sterile 
varieties of dewberries produce an abundance and in some cases, as bigb 
a percentage of morphologically normal pollen grains as any of those 
varieties that are self-fertile, the question of the viability of these 
apparently normal grains becomes of prime importance. 
Pollen was gathered from all of the varieties, and subjected to a 
germination test in the laboratory by means of “hanging-drop” slides 
and germination solutions. The culture medium, as was discovered, 
might consist of either weak sugar solutions of varying strength, tap 
water, or even distilled water. Ordinary tap water was as good as any, 
and less troublesome to secure. 
All of the examined slides disclosed the fact that the pollen grains 
which morphologicallly appeared normal were capable of a vigorous ger¬ 
mination and growth, and that the self-sterility of varieties of dew¬ 
berries is not caused by morphological defectiveness or a lack of via¬ 
bility in the pollen grains. Sterility has its seat in a more remote cause 
and requires further investigation. 
Natural Antipathy Between Pistils and Pollen from the Same 
Variety. —Because self-sterile varieties of dewberries and their hybrids 
will, under normal conditions for cross-pollination, set an abundance 
of fruit, the fact is quite patent that the morphology of the pistils 
(stigma, style, and ovary) is quite normal, and apparently offers no 
explanation of the phenomenon of sterility. From the foregoing recorded 
results in regard to the viability of pollen from self-sterile varieties, 
and also from the good fruit crops secured under ordinary field con¬ 
ditions, we naturally draw the conclusion that the viability of the pollen 
grains is not responsible for the sterility of the variety. All of the 
essential organs of the flowers under certain conditions seem to func¬ 
tion normally, and yet this phenomenon, self-sterility, does exist. 
Whether in the case of self-pollination the pollen functions normally 
and the pistil behaves abnormally, or vice versa, or whether both organs 
refuse to act, has not been determined. At any rate, the antipathy be¬ 
tween the pistil and pollen of the same variety seems to be the immediate 
cause for the failure of fruit setting, and therefore of self-sterility. 
What this rather vague term, natural antipathy, really signifies— 
whether it is due to a toxic substance located in the pollen grain in the 
stigmatic secretions, style, or the ovules—remains yet to be discovered. 
Viable pollen grains from a self-sterile variety, although ineffective 
on the pistils of flowers of the same variety, will bring about perfect 
fertilization when placed on the stigmas of another variety; and it 
matters little whether this second variety be self-fertile or self-sterile, 
of the same species or of another species ( Rubus villosus). 
The following table shows the results of extensive cross-pollination 
tests: 
