These differences have sometimes 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 case, for in some Compositae 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 correlation, 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.