FORMS OF FLOWERS, POLLINATION
When it comes to heterostyly, where flower morphs differing in relative stamen and pistil length are produced in the same species (termed pin and thrum morphs), the coffee family is the champ, with more heterostylous genera around the world than any other plant family. Most are tropical—Darwin worked on seventeen genera in the family, having received plants, seeds, and occasionally dried pressed flowers from friends and colleagues far and wide. Patridgeberry, Mitchella repens, was the only temperate plant he worked on in the family. An evergreen creeper of eastern North American forests, this species was named by Carl Linnaeus in honor of eighteenth-century Virginia botanist and physician John Mitchell, who sent to his European correspondents seeds and specimens of a great many of the “unknown Beautifull, curious and usefull plants, our Country affords.”98
Asa Gray, responding to Darwin’s queries for examples of dimorphic flowers, brought partridgeberry to his attention. “This would be a good plant for you to experiment upon,” he wrote, pointing out its fully dioecious relatives, with their separate-sex flowers.99 Darwin was sure that such floral dimorphisms, which he first noticed with Primula (see p. 271), represented a step in the evolution of separate sexes in flowering plants, and he performed crossing experiments to determine the inter-fertility of different morphs.
Gray sent live partridgeberry specimens in 1862, and when they arrived, Darwin enthused that they “looked as fresh as if dug up the day before! What a pretty little creeper it is with scarlet berries.”100 The fruit, actually a drupe, results from the fusion of the ovaries of the paired trumpet-shaped flowers. In experiments conducted over two seasons, Darwin confirmed that between-morph crosses had a far higher success rate than those within the same morph. But he also noted that partridgeberry was found to occur in functionally dioecious forms as well, with imperfectly formed pistils in one and rudimentary stamens in the other—de facto female and male flowers, even though they technically had the reproductive structures of both sexes. Citing the observations of Thomas Meehan,101 a British-born botanist who started a nursery in the United States, Darwin declared that “Should these statements be confirmed, Mitchella will be proved to be heterostyled in one district and dioecious in another,” and he argued in Forms of Flowers that Mitchella and relatives in the Rubiaceae family thus provided evidence that dioecious species “owe their origin to the transformation of heterostyled plants.”102
In a twist that would have intrigued Darwin, twentieth-century botanists developed a technique for identifying “functional” gender based on the relative seed production of pin versus thrum morphs. In its native habitat, partridgeberry populations vary in functional gender, and these do not necessarily correspond to apparent gender based on morph—stamen-dominated thrums in some cases produce more seeds than pistil-dominated pins.103 Modern thinking is thus partially in agreement with Darwin—heterostyly is an adaptation to promote outcrossing—but does not hold that the morphs necessarily represent an evolution toward dioecy.
Mitchella repens.—Prof. Asa Gray sent me several living plants collected when out of flower, and nearly half of these proved long-styled, and the other half short-styled. The white flowers, which are fragrant and which secrete plenty of nectar, always grow in pairs with their ovaries united, so that the two together produce “a berry-like double drupe.” In my first series of experiments (1864), I did not suppose that this curious arrangement of the flowers would have any influence on their fertility; and in several instances, only one of the two flowers in a pair was fertilised; and a large proportion or all of these failed to produce berries. In the ensuing year, both flowers of each pair were invariably fertilised in the same manner; and the latter experiments alone serve to show the proportion of flowers which yield berries, when legitimately and illegitimately fertilised; but for calculating the average number of seeds per berry I have used those produced during both seasons.
In the long-styled flowers, the stigma projects just above the bearded throat of the corolla, and the anthers are seated some way down the tube. In the short-styled flowers, these organs occupy reversed positions. In this latter form, the fresh pollen-grains are a little larger and more opaque than those of the long-styled form. …
… 88 per cent of the paired flowers of both forms, when legitimately fertilised, yielded double berries, nineteen of which contained on an average 4.4 seeds, with a maximum in one of 8 seeds. Of the illegitimately fertilised paired flowers, only 18 per cent yielded berries, six of which contained on an average only 2.1 seeds, with a maximum in one of 4 seeds. Thus the two legitimate unions are more fertile than the two illegitimate, according to the proportion of flowers which yielded berries, in the ratio of 100 to 20; and according to the average number of contained seeds as 100 to 47.
Three long-styled and three short-styled plants were protected under separate nets, and they produced altogether only 8 berries, containing on an average only 1.5 seed. Some additional berries were produced which contained no seeds. The plants thus treated were therefore excessively sterile, and their slight degree of fertility may be attributed in part to the action of the many individuals of Thrips which haunted the flowers. …
… In the ‘Genera Plantarum,’ by Bentham and Hooker, the Rubiaceae are divided into 25 tribes, containing 337 genera; and it deserves notice that the genera now known to be heterostyled are not grouped in one or two of these tribes, but are distributed in no less than eight of them. From this fact we may infer that most of the genera have acquired their heterostyled structure independently of one another; that is, they have not inherited this structure from some one or even two or three progenitors in common.
Orchis apifera. Water and bodycolor on vellum by English School artist, Album of Garden Flowers.