Even with the individuals of
the same species, as just remarked, some are utterly self-sterile,
others moderately so, and some perfectly self-fertile. The cause,
whatever it may be, which renders many plants more or less sterile with
their own pollen, that is, when they are self-fertilised, must be
different, at least to a certain extent, from that which determines the
difference in height, vigour, and fertility of the seedlings raised from
self-fertilised and crossed seeds; for we have already seen that the two
classes of cases do not by any means run parallel. This want of
parallelism would be intelligible, if it could be shown that
self-sterility depended solely on the incapacity of the pollen-tubes to
penetrate the stigma of the same flower deeply enough to reach the
ovules; whilst the greater or less vigorous growth of the seedlings no
doubt depends on the nature of the contents of the pollen-grains and
ovules. Now it is certain that with some plants the stigmatic secretion
does not properly excite the pollen-grains, so that the tubes are not
properly developed, if the pollen is taken from the same flower. This is
the case according to Fritz Muller with Eschscholtzia, for he found that
the pollen-tubes did not penetrate the stigma deeply; and with the
Orchidaceous genus Notylia they failed altogether to penetrate it.
(9/14. 'Botanische Zeitung' 1868 pages 114, 115.
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