The cause of petaloid colouring in ‘Apetalous’ Flowers

SUMMARY The typical dichlamydeous cyclic Dicotyledon is so organised that the petaloid character of the corolla can be regarded as a function of a certain combination of conditions as regards time and space: time, in so far that the petaloid feature occurs at a definite stage in the series of developmental processes, following, as it does, upon the differentiation of a (usually) uncoloured (i.e. green) calyx; space, in that it is exhibited on the set of radii alternating with the radii of the sepals. Penetrating a little below the surface appearance, we find we can express these same relations in terms of the vascular anatomy as follows. Those floral members (again taking the typical case) which receive as midribs the first set of equidistant bundles to leave the central cylinder show sepaloid characters; those similarly receiving as midrib bundles the outgoing strands on the alternate set of radii exhibit petaloid colouring. It is found that the marginal veins of the sepals of such Dicotyledon types arise in two different ways, less frequently as true lateral veins from the midribs, more often either through the detachment from the central cylinder on the alternate radii of trunk cords which dissociate in due course into a petal midrib and twin bundles which enter the adjacent side of the sepal to right and left, respectively, and become the marginal vein of that side; or through the departure of pairs of separate strands within the limits of the corresponding alternate sectors. In typical monochlamydeous cyclic Dicotyledons radial organisation follows the same scheme as in dichlamydeous types, notwithstanding that the perianth here takes the form of a single whorl of structures. Such monochlamydeous types may be divided into two classes. In the one class only the issuing vascular bundles on the corresponding set of radii enter the perianth members. These bundles become the midribs. They may give off lateral veins at any point or may remain unbranched. In either case the individual member is homologous with the individual sepal and is typically green. In the other class each member receives not only the bundle on its own radius but also half the perianth component proper to the alternate radius on each side, either as separate strands or (in gamophyllous types) as undisjoined components of perianth-stamen trunk cords. The first-mentioned bundle becomes the midrib of the tepal, the two others become marginal veins, the one entering the tepal on the right, the other that on the left. In forms belonging to this class the tepals are typically petaloid. Each may be regarded as the counterpart of one sepal of dichlamydeous types + half the neighbouring petal on either side. This equivalence is not infrequently indicated outwardly by the considerable thickness of the tepal members as compared with that of the sepals and petals of the nearest allied dichlamydeous forms. The accompanying drawings were made by Miss D. F. M. Pertz, to whom I desire to express my grateful thanks.