Of their cause we are quite ignorant; we cannot even attribute them, as in the last class of cases, to any proximate agency, such as relative position.

I will give only a few instances.

It is so common to observe on the same plant, flowers indifferently tetramerous, pentamerous, etc., that I need not give examples; but as numerical variations are comparatively rare when the parts are few, I may mention that, according to De Candolle, the flowers of Papaver bracteatum offer either two sepals with four petals (which is the common type with poppies), or three sepals with six petals.

The manner in which the petals are folded in the bud is in most groups a very constant morphological character; but Professor Asa Gray states that with some species of Mimulus, the aestivation is almost as frequently that of the Rhinanthideae as of the Antirrhinideae, to which latter tribe the genus belongs.Aug.

St.Hilaire gives the following cases: the genus Zanthoxylon belongs to a division of the Rutaceae with a single ovary, but in some species flowers may be found on the same plant, and even in the same panicle, with either one or two ovaries.
In Helianthemum the capsule has been described as unilocular or tri-locular; and in H.mutabile, "Une lame PLUS OU MOINS LARGE, s'etend entre le pericarpe et le placenta." In the flowers of Saponaria officinalis Dr.Masters has observed instances of both marginal and free central placentation.