As in all these cases
the stigmas of the short-styled pistil are seated low down within a more or less
tubular corolla, it is probable that they are better fitted by being long and
narrow for brushing the pollen off the inserted proboscis of an insect.
With many heterostyled plants the stigma differs in roughness in the two forms,
and when this is the case there is no known exception to the rule that the
papillae on the stigma of the long-styled form are longer and often thicker than
those on that of the short-styled. For instance, the papillae on the long-styled
stigma of Hottonia palustris are more than twice the length of those in the
other form. This holds good even in the case of Houstonia coerulea, in which the
stigmas are much shorter and stouter in the long-styled than in the short-styled
form, for the papillae on the former compared with those on the latter are as
100 to 58 in length. The length of the pistil in the long-styled form of Linum
grandiflorum varies much, and the stigmatic papillae vary in a corresponding
manner. From this fact I inferred at first that in all cases the difference in
length between the stigmatic papillae in the two forms was one merely of
correlated growth; but this can hardly be the true or general explanation, as
the shorter stigmas of the long-styled form of Houstonia have the longer
papillae.
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