Lysimachia vulgaris, Euphrasia officinalis, Rhinanthus
crista-galli, and Viola tricolor come under this head. (Introduction/4. H.
Muller 'Nature' September 25, 1873 volume 8 page 433 and November 20, 1873
volume 9 page 44. Also 'Die Befruchtung der Blumen' etc. 1873 page 294.) The
smaller and less conspicuous flowers are not closed, but as far as the purpose
which they serve is concerned, namely, the assured propagation of the species,
they approach in nature cleistogamic flowers; but they differ from them by the
two kinds being produced on distinct plants.
With many plants, the flowers towards the outside of the inflorescence are much
larger and more conspicuous than the central ones. As I shall not have occasion
to refer to plants of this kind in the following chapters, I will here give a
few details respecting them. It is familiar to every one that the ray-florets of
the Compositae often differ remarkably from the others; and so it is with the
outer flowers of many Umbelliferae, some Cruciferae and a few other families.
Several species of Hydrangea and Viburnum offer striking instances of the same
fact. The Rubiaceous genus Mussaenda presents a very curious appearance from
some of the flowers having the tip of one of the sepals developed into a large
petal-like expansion, coloured either white or purple.
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