onclude that no general rule
can be laid down; but any great deviation from the normal structure,
even when the reproductive organs themselves are not seriously
affected, certainly often leads to sexual impotence.

_Double Flowers._--When the stamens are converted into petals, the
plant becomes on the male side sterile; when both stamens and pistils
are thus changed, the plant becomes completely barren. Symmetrical
flowers having numerous stamens and petals are the most liable to
become double, as perhaps follows from all multiple organs being the
most subject to variability. But flowers furnished with only a few
stamens, and others which are asymmetrical in structure, sometimes
become double, as we see with the double gorse or Ulex, Petunia, and
Antirrhinum. The Compositae bear what are called double flowers by the
abnormal development of the corolla of their central florets.
Doubleness is sometimes connected with prolification,[414] or the
continued growth of the axis of the flower. Doubleness is strongly
inherited. No one has produced, as Lindley remarks,[415] double flowers
by promoting the perfect health of the plant. On the contrary,
unnatural conditions of life favour their production. There is some
reason to believe that seeds kept during many years, and seeds believed
to be imperfectly fertilised, yield double flowers more freely than
fresh and perfectly fertilised seed.[416] Long-continued cultivation in
rich soil seems to be the commonest exciting cause. A double narcissus
and a double _Anthemis nobilis_, transplanted into very poor soil, have
been observed to become single;[417] and I have seen a completely
double white primrose rendered permanently single by being divided and
transplanted whilst in full flower. It has been observed by Professor
Morren that doubleness of the flowers and variegation of the leaves are
antagonistic states; but so many exceptions to the rule have lately
been recorded,[418] that, though general, it cannot be looked at as
invariable. {168} Variegation seems generally to result from a feeble
or atrophied condition of the plant, and a large proportion of the
seedlings raised from parents both of which are variegated usually
perish at an early age; hence we may perhaps infer that doubleness,
which is the antagonistic state, commonly arises from a pleth