Possibly
these several differences may be connected with the different flow of
nutriment towards the central and external flowers. We know, at least,
that with irregular flowers those nearest to the axis are most subject to
peloria, that is to become abnormally symmetrical. I may add, as an
instance of this fact, and as a striking case of correlation, that in many
pelargoniums the two upper petals in the central flower of the truss often
lose their patches of darker colour; and when this occurs, the adherent
nectary is quite aborted, the central flower thus becoming peloric or
regular. When the colour is absent from only one of the two upper petals,
the nectary is not quite aborted but is much shortened.
With respect to the development of the corolla, Sprengel's idea that the
ray-florets serve to attract insects, whose agency is highly advantageous,
or necessary for the fertilisation of these plants, is highly probable; and
if so, natural selection may have come into play. But with respect to the
seeds, it seems impossible that their differences in shape, which are not
always correlated with any difference in the corolla, can be in any way
beneficial; yet in the Umbelliferae these differences are of such apparent
importance--the seeds being sometimes orthospermous in the exterior flowers
and coelospermous in the central flowers--that the elder De Candolle
founded his main divisions in the order on such characters.
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