satisfactorily explained, is scarcely to be expected: the circumstances of each case have to be studied. But it seems not improbable that they are due to causes of the kind indicated.[36] § 235. We have already glanced at clusters of flowers for the purpose of considering their shapes as clusters. We must now return to them to observe the modifications undergone by their component flowers. Among these occur illustrations of great significance. An example of transition from the radial to the bilateral form in clustered flowers of the same species, is furnished by the cultivated _Geraniums_, called by florists _Pelargoniums_. Some of these, bearing somewhat small terminal clusters of flowers, which are closely packed together with their faces almost upwards, have radially-symmetrical flowers. But among other varieties having terminal clusters of which the members are mutually thrust on one side by crowding, the flowers depart very considerably from the radial shape towards the bilateral shape. A like result occurs under like conditions in Rhododendrons and Azaleas. The _Verbena_, too, furnishes an illustration of radial flowers rendered slightly two-sided by the slight two-sidedness of their relations to other flowers in the cluster. And among the _Cruciferæ_ a kindred case occurs in the cultivated Candytuft. Evidence of a somewhat different kind is offered us by clustered flowers in which the peripheral members of the clusters differ from the central members; and this evidence is especially significant where we find allied species that do not exhibit the deviation, at the same time that they do not fulfil the conditions under which it may be expected. Thus, in _Scabiosa succisa_, Fig. 250, which bears its numerous small flowers in a hemispherical knob, the component flowers, similarly circumstanced, are all equal and all radial; but in _Scabiosa arvensis_, Fig. 251, in which the numerous small flowers form a flattened disk only the confined central ones are radial: round the edge the flowers are much larger and conspicuously bilateral. [Illustration: Figs. 250, 251.] [Illustration: Fig. 252.]