On the other hand, the flowers of the common potato (S.
tuberosum), though they do not secrete nectar Kurr 'Bedeutung der
Nektarien' 1833 page 40, yet cannot be considered as inconspicuous, and
they are sometimes visited by diptera (Muller), and, as I have seen, by
humble-bees. Tinzmann (as quoted in 'Gardeners' Chronicle' 1846 page
183, found that some of the varieties did not bear seed when fertilised
with pollen from the same variety, but were fertile with that from
another variety.)
It should be remembered that in two of the cases in which highly
self-fertile varieties appeared amongst my experimental plants, namely,
with Mimulus and Nicotiana, such varieties were greatly benefited by a
cross with a fresh stock or with a slightly different variety; and this
likewise was the case with the cultivated varieties of Pisum sativum and
Lathyrus odoratus, which have been long propagated by
self-fertilisation. Therefore until the contrary is distinctly proved, I
must believe that as a general rule small and inconspicuous flowers are
occasionally intercrossed by insects; and that after long-continued
self-fertilisation, if they are crossed with pollen brought from a plant
growing under somewhat different conditions, or descended from one thus
growing, their offspring would profit greatly. It cannot be admitted,
under our present state of knowledge, that self-fertilisation continued
during many successive generations is ever the most beneficial method of
reproduction.
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