y verticillate.[434]

[Illustration: FIG. 190.--Double white lily. Multiplication of
perianth-segments and other changes.]

=Pleiotaxy of the corolla.=--With reference to double flowers, it was
remarked by Linne that polypetalous flowers were, as he said,
multiplied, while monopetalous flowers were duplicated, or triplicated,
as the case may be,[435] a statement that is true in the main, though
it requires modification. In the case of polypetalous, or rather
dialypetalous flowers, the petals may be very largely increased by
multiplication, as in roses, anemones, pinks, &c. In the last-named
genus the number is often so much increased that the calyx splits from
the tension exercised on it by the increasing mass within. This
multiplication may happen without any metamorphy or substitution of
petals for stamens, though, in the majority of cases, it is associated
with such a change. It is curious to observe in some of these flowers
that the total number of parts is not greatly increased; thus, in some
of the double-flowered _Leguminosae_, such as _Ulex europaeus_ and _Lotus
corniculatus_, the petals are repeated once or twice, the stamens are
petalodic, but reduced in number, while the carpels are usually entirely
wanting. Thus, owing to the diminished number of parts in the inner
whorls of the flower, these very double-looking blooms do not contain
any greatly increased number of parts.[436]

Flowers that, under ordinary circumstances, are gamopetalous, become, in
some instances, multiplied by the formation of additional segments, just
as in the case of polypetalous corollas; but in these cases the corollas
become polypetalous, their petals do not cohere one with another. Among
double flowers of this character may be mentioned _Campanula
rotundifolia_, _Gardenia_ sp., _Nerium Oleander_, _Serissa_ sp.,
_Arbutus Unedo_, &c. The change is associated with petalody of the
stamens and pistils.

A more frequent change among the monopetalous orders is the duplication
or triplication of the corolla, in consequence of which there appear to
be a series of corollas enclosed one within the other, the lobes of
which generally alternate with one another, but which sometimes are
superposed. This happens occasionally in the primrose (_Primula
acaulis_), and constitutes the variety called by the gardeners "hose in
hose."

The same condition occurs frequently in some species of _Datura_ and
_Campanula_.

[Illustration: FIG. 191.--_Ca