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CHAPTER IX.

GRADATION OF ORGANS, &C.—CONCLUDING REMARKS.

Gradation of organs, of the rostellum, of the pollen-masses—Formation of the caudicle—Genealogical affinities—Secretion of nectar—Mechanism of the movement of the pollinia—Uses of the petals—Production of seed—Importance of trifling details of structure—Cause of the great diversity of structure in the flowers of Orchids—Cause of the perfection of the contrivances—Summary on insect-agency—Nature abhors perpetual self-fertilisation.

This chapter will be devoted to the consideration of several miscellaneous subjects which could not well have been introduced elsewhere.

On the gradation of certain Organs.—The rostellum, the pollinia, the labellum, and, in a lesser degree, the column, are the most remarkable points in the structure of Orchids. The formation of the column and labellum, by the confluence and partial abortion of several organs, has been discussed in the last chapter. With respect to the rostellum, no such organ exists in any other group of plants. If the homologies of Orchids had not been pretty well made out, those who believe in the separate creation of each organism might have advanced this as an excellent instance of a perfectly new organ having been specially created, and which could not have been developed by successive slow modifications of any pre-existing part. But, as Robert Brown long ago remarked, it is not a new organ. It is impossible to look at the two groups of spiral vessels (fig. 36) running from the bases of