Our sole means for forming any judgment on this head is the duration of
the varieties of our fruit trees which have been long propagated by
grafts or buds. Andrew Knight formerly maintained that under these
circumstances they always become weakly, but this conclusion has been
warmly disputed by others. A recent and competent judge, Professor Asa
Gray, leans to the side of Andrew Knight, which seems to me, from such
evidence as I have been able to collect, the more probable view,
notwithstanding many opposed facts. (12/4. 'Darwiniana: Essays and
Reviews pertaining to Darwinism' 1876 page 338.)
The means for favouring cross-fertilisation and preventing
self-fertilisation, or conversely for favouring self-fertilisation and
preventing to a certain extent cross-fertilisation, are wonderfully
diversified; and it is remarkable that these differ widely in closely
allied plants,--in the species of the same genus, and sometimes in the
individuals of the same species. (12/5. Hildebrand has insisted strongly
to this effect in his valuable observations on the fertilisation of the
Gramineae: 'Monatsbericht K. Akad. Berlin' October 1872 page 763.) It is
not rare to find hermaphrodite plants and others with separated sexes
within the same genus; and it is common to find some of the species
dichogamous and others maturing their sexual elements simultaneously.
The dichogamous genus Saxifraga contains proterandrous and proterogynous
species.
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