Page - 157 - in The Origin of Species

CORRELATED VARIATION 157 difference between the outer and inner flowers in some Com- positous and Umbelliferous plants. Every one is familiar with the difference between the ray and central florets of, for instance, the daisy, and this dift'erence is often accom- panied with the partial or complete abortion of the repro- ductive organs. But in some of these plants, the seeds also differ in shape and sculpture. These differences have some- times been attributed to the pressure of the involucra on the florets, or to their mutual pressure, and the shape of the seeds in the ray-florets of some Compositae countenances this idea; but with the Umbelliferae, it is by no means, as Dr. Hooker informs me, the species with the densest heads which most frequently differ in their inner and outer flowers. It might have been thought that the development of the ray- petals by drawing nourishment from the reproductive organs causes their abortion; but this can hardly be the sole cause, for in some Compositze the seeds of the outer and inner florets differ, without any difference in the corolla. Possibly these several differences may be connected with the different flow of nutriment towards the central and external flowers : we know, at least, that with irregular flowers, those nearest to the axis are most subject to peloria, that is to become abnormally symmetrical. I may add, as an instance of this fact, and as a striking case of correla- tion, that in many pelargoniums, the two upper petals in the central flower of the truss often lose their patches of darker colour; and when this occurs, the adherent nectary is quite aborted; the central flower thus becoming peloric or regular. When the colour is absent from only one of the two upper petals, the nectary is not quite aborted but ismuch shortened. With respect to the development of the corolla, Sprengel's idea that the ray-florets serve to attract insects, whose agency is highly advantageous or necessary for the fertili- sation of these plants, is highly probable; and if so. natural selection may have come into play. But with respect to the seeds, it seems impossible that their differences in shape, which are not always correlated with any difference in the corolla, can be in any way beneficial: yet in the UmbcUifera; these differences are of such apparent importance —the seeds being sometimes orthospermous in the exterior flowers and