These have been observed in three families, and consist of
species of Lythrum and of the allied genus Nesaea, of Oxalis and Pontederia. In
their manner of fertilisation these plants offer a more remarkable case than can
be found in any other plant or animal.
Lythrum salicaria.
(FIGURE 4.10. Diagram of the flowers of the three forms of Lythrum salicaria, in
their natural position, with the petals and calyx removed on the near side:
enlarged six times.
Top: Long-styled.
Middle: Mid-styled.
Bottom: Short-styled.
The dotted lines with the arrows show the directions in which pollen must be
carried to each stigma to ensure full fertility.)
The pistil in each form differs from that in either of the other forms, and in
each there are two sets of stamens different in appearance and function. But one
set of stamens in each form corresponds with a set in one of the other two
forms. Altogether this one species includes three females or female organs and
three sets of male organs, all as distinct from one another as if they belonged
to different species; and if smaller functional differences are considered,
there are five distinct sets of males. Two of the three hermaphrodites must
coexist, and pollen must be carried by insects reciprocally from one to the
other, in order that either of the two should be fully fertile; but unless all
three forms coexist, two sets of stamens will be wasted, and the organisation of
the species, as a whole, will be incomplete.
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