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Show 286 DHECIOUS AND CHAP. VII. ovarium, surmounted by a mere vestige of a stigma without any style; whilst the female flowers possess a large ovarium, the anthers being rudimentary and apparently quite destitute of pollen. Considering how many Rubiaceous genera are heterostyled, it is a reasonable suspicion that this Asperula is descended from a heterostyled progenitor; but we should be cautious on this head, for there is no improbability in a homostyled Rubiaceous plant becoming dicecious. Moreover, in . an allied plant, Galium cruciatum, the female organs have been suppressed in most of the lower flowers, whilst the upper ones remain hermaphrodite; and here we have a modification of the sexual organs without any connection with heterostylism. Mr. Thwaites informs me that in Ceylon various Rubiaceous plants are heterostyled; but in the case of Discospermum one of the two forms is always barren, the ovary containing about two aborted ovules in each loculus·; whilst in the other form each loculus contains several perfect ovules ; so that the species appears to be strictly dimcious. Most of the species of the South American genus lEgiphila, a member of the Verbenacere, apparently are heterostyled; and both ~-,ritz l\iiiller and myself thought that this was the case with .lE. obdurata, so closely did its flowers resemble those of the heterosty led species. But on examining the flowers, the anthers of the long-styled form were found to be entirely destitute of pollen and less than half the size of those in the other form, the pistil being perfectly developed. On the other hand, in the short-styled form the stigmas are reduced to half their proper length, having also an abnorn1al appearance; whilst the stamens are perfect. This plant therefore is direcious ; and we may, I think, conclude that a short-styled progenitor, CHAP. VII. POLYGAMOUS PLANTS. 287. bearing long stamens exserted beyond the corolla, has been converted into the male; and a long-styled progenitor with fully developed stigmas into the female. From the number of bad pollen-grains in the small anthers of the short stamens of the long-sty led form of Pulmonaria angustijolia, we may suspect that this form is tending to become female; but it does not appear that the other or short-styled form is becoming more masculine. Certain appearances countenance the belief that the reproductive system of Phlox subulata is likewise undergoing a change of some kind. I have now given the few cases known to me in which heterostyled plants appear with some considerable degree of probability to have been rendered dioocious. Nor ought we to expect to find many such cases, for the number of heterostyled species is by no means large, at least in Europe, where they could hardly have escaped notice. Therefore the number of dioocious species which owe their origin to the transformation of heterostyled plants is probably not so large as might have been anticipated from the facilities which they offer for such conversion. In searching for cases like the foregoing ones, I have been led to examine some dimcious or sub-dimcious plants, which are worth describing, chiefly as they show. by what fine gradations hermaphrodites may pass Into polygamous or dimcious species. Polygamous, Dimcious and Sub-dimcious Plants. . Euon¥mus "l!uropmus ( Celastrinere) .-The spindle-tree Is descnbed 1n all the botanical works which I have consulted as an hermaphrodite. Asa Gray speaks of the flowers of the American species as perfect, whilst |