Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/255

. VIII. the whorls, R. Brown offers no sufficient evidence, but believes that they are combined with the labellum, whenever that organ presents crests or ridges. In these views Brown is followed by Lindley.

Brown traced the spiral vessels in the flower by making transverse sections, and only occasionally, as far as it appears, by longitudinal sections. As spiral vessels are developed at a very early period of growth, and this circumstance always gives much value to a part in making out homologies; and as they are apparently of high functional importance, though their function is not well known, it appeared to me, guided also by the advice of Dr. Hooker, to be worth while to trace upwards all the spiral vessels from the six groups surrounding the ovarium. Of the six ovarian groups of vessels, I will call (though not correctly) that under the labellum the anterior group; that under the upper sepal the posterior group; and the two groups on the two sides of the ovarium the antero-lateral and postero-lateral groups.

The result of my dissections is given in the following diagram (fig. 36). The fifteen little circles represent