CROSS AND SELF-FERTILIZATION
Some 150 species are classified in the genus Linaria, a temperate European, African, and Asian group in a family related to snapdragons. The Latin name, and the latter half of the common name, refers to the similarity of the leaves of those of many flax plants (see p. 121). Darwin grew several species of Linaria in his experimental gardens, mainly as part of his research into the effects of self-fertilization versus outcrossing in flowering plants. One set of studies involved common toadflax, Linaria vulgaris, a European species now with a cosmopolitan distribution, also called butter-and-eggs due to its lovely yellow and orange blossoms. In the course of his investigations into the role of insects as pollen-carrying go-betweens, Darwin found that without insect aid, the flowers of L. vulgaris are sterile for the most part, with minimal seed production; 100 seeds yielded by cross-fertilized flowers compared to just 14 seeds from self-pollinated ones. What’s more, the seedlings grown from the few seeds that were produced by self-pollination tended to be shorter and less vigorous than those resulting from outcrossed flowers.
It often occurred to me that it would be advisable to try whether seedlings from cross-fertilised flowers were in any way superior to those from self-fertilised flowers. But as no instance was known with animals of any evil appearing in a single generation from the closest possible interbreeding, that is between brothers and sisters, I thought that the same rule would hold good with plants; and that it would be necessary at the sacrifice of too much time to self-fertilise and intercross plants during several successive generations, in order to arrive at any result. I ought to have reflected that such elaborate provisions favouring cross-fertilisation, as we see in innumerable plants, would not have been acquired for the sake of gaining a distant and slight advantage, or of avoiding a distant and slight evil. Moreover, the fertilisation of a flower by its own pollen corresponds to a closer form of interbreeding than is possible with ordinary bi-sexual animals; so that an earlier result might have been expected.
I was at last led to make the experiments recorded in the present volume from the following circumstance. For the sake of determining certain points with respect to inheritance, and without any thought of the effects of close interbreeding, I raised close together two large beds of self-fertilised and crossed seedlings from the same plant of Linaria vulgaris. To my surprise, the crossed plants when fully grown were plainly taller and more vigorous than the self-fertilised ones. Bees incessantly visit the flowers of this Linaria and carry pollen from one to the other; and if insects are excluded, the flowers produce extremely few seeds; so that the wild plants from which my seedlings were raised must have been intercrossed during all previous generations. It seemed therefore quite incredible that the difference between the two beds of seedlings could have been due to a single act of self-fertilisation; and I attributed the result to the self-fertilised seeds not having been well ripened, improbable as it was that all should have been in this state, or to some other accidental and inexplicable cause. …
The trial was afterwards repeated with more care; but as this was one of the first plants experimented on, my usual method was not followed. Seeds were taken from wild plants growing in this neighbourhood and sown in poor soil in my garden. Five plants were covered with a net, the others being left exposed to the bees, which incessantly visit the flowers of this species, and which, according to H. Müller, are the exclusive fertilisers, This excellent observer remarks that, as the stigma lies between the anthers and is mature at the same time with them, self-fertilisation is possible. But so few seeds are produced by protected plants, that the pollen and stigma of the same flower seem to have little power of mutual interaction. The exposed plants bore numerous capsules forming solid spikes. Five of these capsules were examined and appeared to contain an equal number of seeds; and these being counted in one capsule were found to be 166. The five protected plants produced altogether only twenty-five capsules, of which five were much finer than all the others, and these contained an average of 23.6 seeds, with a maximum in one capsule of fifty-five. So that the number of seeds in the capsules on the exposed plants to the average number in the finest capsules on the protected plants was as 100 to 14.
Some of the spontaneously self-fertilised seeds from under the net, and some seeds from the uncovered plants naturally fertilised and almost certainly intercrossed by the bees, were sown separately in two large pots of the same size; so that the two lots of seedlings were not subjected to any mutual competition. Three of the crossed plants when in full flower were measured, but no care was taken to select the tallest plants; their heights were 74\8, 72\8, and 64\8 inches; averaging 7.08 in height. The three tallest of all the self-fertilised plants were then carefully selected, and their heights were 6⅜, 55\8, and 52\8, averaging 5.75 in height. So that the naturally crossed plants were to the spontaneously self-fertilised plants in height, at least as much as 100 to 81.
Modern studies show that the largely self-incompatible flowers of L. vulgaris employ several strategies to command the attention of pollinators, including nectaries (that conspicuous spur where nectar collects) and olfactory and visual attractants.85
Darwin also observed bees visiting the beautiful purple toadflax (L. purpurea), a tall species native to the Italian peninsula and now grown in gardens worldwide. The uniform coloring of these flowers cast doubt, for him, on the suggestion made by German botanist Konrad Sprengel in 1793 that streaks and stripes on petals played a role in attracting or directing insect visitors, facilitating pollination. Darwin remarked in a letter published in the Gardeners’ Chronicle that he knew “hardly any flower which bees open and insert their proboscis into, more rapidly, than the common tall linaria, which has a little purplish well-closed flower; I have watched one humble-bee suck twenty-four flowers in one minute, yet on this flower there are no streaks of colour to guide these quick and clever workmen.”86
Sprengel’s hypothesis is largely accepted today, with such streaks and stripes now termed “nectar guides,” and an understanding that many flowers which appear uniform in color to our eyes actually do sport them, but in the ultraviolet end of the spectrum that bees can perceive. Darwin would be fascinated to learn that nectar guides may in fact reduce the nectar-robbery he often observed bumble bees engaging in, by reducing the bees’ handling time and access to the business end of a flower—a double benefit to the plant, since reduced nectar robbery translates into increased pollination.87
Linum perenne. Water and bodycolor on vellum by Dame Ann Hamilton, Drawings of Plants.