CROSS AND SELF-FERTILIZATION
The genus Dianthus, derived from Greek terms meaning “divine flower” for its beauty and fragrance, includes around 300 species native mainly to Europe and Asia. Several Dianthus species were prized by the ancients, and two of them played a starring role in horticultural history when, in 1717, the renowned English horticulturist Thomas Fairchild produced what may be the first experimental hybrid plant by crossing D. caryophyllus (Carnation) with D. bartatus (Sweet William).49 “Fairchild’s Mule,” as the hybrid was called, was widely celebrated at the time—Erasmus Darwin, Charles’s grandfather, waxed poetic over the feat in The Loves of the Plants. At a time when the nature of reproduction in plants and Carl Linnaeus’s sex-based botanical classification system were hotly debated, Erasmus and others cited Fairchild’s Mule as evidence in support of the Swedish savant. The sterility of “vegetable mules,” Erasmus noted, “supply an irrefragable argument in favour of the sexual system of botany.”50
Carnations were especially popular in Darwin’s era, and their popularity continues—today the International Dianthus Register of the Royal Horticultural Society lists over 30,000 cultivars. Darwin began his interest in crossbreeding of carnation flowers in 1855, requesting seeds from his college mentor John Stevens Henslow to try experiments on hybridization. He asked for seeds of “wild” Dianthus caryophyllus but it was hardly a wild plant, with its exact range in nature unknown since it had been cultivated for more than 2000 years.
Soon after he studied pollination in orchid flowers, Darwin began systematic self- and cross-fertilization experiments on many genera of plants. It all started with an observation he first made with Linaria (see p. 185) when he planted, side-by-side, beds of seedlings produced by selfing versus crossing. “To my surprise, the crossed plants when fully grown were plainly taller and more vigorous than the self-fertilised ones,” he marveled. “During the next year, I raised for the same purpose as before two large beds close together of self-fertilised and crossed seedlings from the carnation, Dianthus caryophyllus.” The result was the same. His attention “now thoroughly aroused,” he set about making thousands of crosses with dozens of plant species, resulting in the publication of The Effects of Cross and Self Fertilisation in the Vegetable Kingdom in 1876.51
Dianthus flowers are termed “protandrous” (Darwin’s “proterandrous”), with the anthers maturing as much as a week before the stigmas become receptive, and their fragrance is not only appealing to people but certainly to the many insects, including humble-bees (a.k.a., bumblebees) that pollinate them with abandon. Darwin’s experiments confirmed the advantage of cross-pollination between Dianthus flowers of different plants rather than on the same plant. He crossed and selfed plants and collected seeds over four generations, measuring the height and weight and counting the number of seeds produced by each plant in each successive generation and observing the flower color and patterns yielded by the crosses. His results confirmed that horticulturalists should rely on cross-pollination—outcrossing—to obtain the most robust and fertile plants.
Dianthus caryophyllus. The common carnation is strongly proterandrous, and therefore depends to a large extent upon insects for fertilisation. I have seen only humble-bees visiting the flowers, but I dare say other insects likewise do so. It is notorious that if pure seed is desired, the greatest care is necessary to prevent the varieties which grow in the same garden from intercrossing. The pollen is generally shed and lost before the two stigmas in the same flower diverge and are ready to be fertilised. I was therefore often forced to use for self-fertilisation pollen from the same plant instead of from the same flower. …
Several single-flowered carnations were planted in good soil and were all covered with a net. Eight flowers were crossed with pollen from a distinct plant and yielded six capsules, containing on an average 88.6 seeds, with a maximum in one of 112 seeds. Eight other flowers were self-fertilised in the manner above described, and yielded seven capsules containing on an average 82 seeds, with a maximum in one of 112 seeds. So that there was very little difference in the number of seeds produced by cross-fertilisation and self-fertilisation, viz., as 100 to 92. As these plants were covered by a net, they produced spontaneously only a few capsules containing any seeds, and these few may perhaps be attributed to the action of Thrips and other minute insects which haunt the flowers. A large majority of the spontaneously self-fertilised capsules produced by several plants contained no seeds, or only a single one. Excluding these latter capsules, I counted the seeds in eighteen of the finest ones, and these contained on an average 18 seeds. …
Crossed and self-fertilised Plants of the First Generation.—The many seeds obtained from the above crossed and artificially self-fertilised flowers were sown out of doors, and two large beds of seedlings, closely adjoining one another, thus raised. This was the first plant on which I experimented, and I had not then formed any regular scheme of operation. When the two lots were in full flower, I measured roughly a large number of plants, but record only that the crossed were on an average fully 4 inches taller than the self-fertilised. Judging from subsequent measurements, we may assume that the crossed plants were about 28 inches, and the self-fertilised about 24 inches in height; and this will give us a ratio of 100 to 86. Out of a large number of plants, four of the crossed ones flowered before any one of the self-fertilised plants.
Thirty flowers on these crossed plants of the first generation were again crossed with pollen from a distinct plant of the same lot, and yielded twenty-nine capsules, containing on an average 55.62 seeds, with a maximum in one of 110 seeds.
Crossed and self-fertilised Plants of the Second Generation.—The crossed and self-fertilised seeds from the crossed and self-fertilised plants of the last generation were sown on opposite sides of two pots; … Some flowers on these crossed plants were again crossed with pollen from another plant of the same lot, and some flowers on the self-fertilised plants again self-fertilised; and from the seeds thus obtained the plants of the next generation were raised.
Crossed and self-fertilised Plants of the Third Generation.—The seeds just alluded to were allowed to germinate on bare sand and were planted in pairs on the opposite sides of four pots. When the seedlings were in full flower, the tallest stem on each plant was measured to the base of the calyx. … In Pot I. the crossed and self-fertilised plants flowered at the same time; but in the other three pots the crossed flowered first. These latter plants also continued flowering much later in the autumn than the self-fertilised.
The average height of the eight crossed plants is here 28.39 inches, and of the eight self-fertilised 28.21; or as 100 to 99. So that there was no difference in height worth speaking of, but in general vigour and luxuriance there was an astonishing difference, as shown by their weights. After the seed-capsules had been gathered, the eight crossed and the eight self-fertilised plants were cut down and weighed; the former weighed 43 ounces, and the latter only 21 ounces; or as 100 to 49. …
In summary: This plant was experimented on during four generations, in three of which the crossed plants exceeded in height the self-fertilised generally by much more than five per cent.; and we have seen that the offspring from the plants of the third self-fertilised generation crossed by a fresh stock profited in height and fertility to an extraordinary degree. But in this third generation the crossed plants of the same stock were in height to the self-fertilised only as 100 to 99, that is, they were practically equal. Nevertheless, when the eight crossed and eight self-fertilised plants were cut down and weighed, the former were to the latter in weight as 100 to 49! There can therefore be not the least doubt that the crossed plants of this species are greatly superior in vigour and luxuriance to the self-fertilised.
Digitalis purpurea. Watercolor on vellum by Lady Frances Howard, A Catalogue of English Plants.