FORMS OF FLOWERS, POLLINATION, CROSS AND SELF-FERTILIZATION
String beans, black beans, pinto beans, scarlet runner beans—all are in the genus Phaseolus, with seventy annual and perennial species, five of which are domesticated with varieties grown as bush beans and pole beans, producing green beans and dry beans. The most commonly grown ones—Phaseolus vulgaris, common bean, and P. coccineus, scarlet runner bean—are both originally from Central America and have been cultivated for millennia.
Darwin surely grew and ate a lot of beans, that staple of the kitchen garden. He first commented on them in connection with pollination in his “Transmutation Notebooks” of the late 1830s and early 1840s, making observations in the gardens at his and his wife Emma’s childhood homes in Shrewsbury and nearby Maer Hall. He was fascinated by the curious structure of bean flowers, with stamens and pistil coiled within the tubular keel-petal (“like a French horn”), below which hang two wing-petals. He delighted in how the stamens and pistil are suddenly exserted to dab pollen on unsuspecting large bumblebees (known at the time as “humble-bees”) that alight on the wing-petals. He grew very fond of the bees, even rallying to their defense when an irate correspondent in the Gardeners’ Chronicle called for their eradication, accusing them of damaging the bean crop by perforating the base of the flower to steal the nectar. Darwin wrote a letter in reply, acknowledging that the bees may be guilty of not extracting nectar “in the manner nature intended them” and so indirectly affecting seed set by failing to pollinate. But he urged forgiveness. “Although I can believe that such wicked bees may be injurious to the seedsman,” Darwin continued, “one would lament to see these industrious, happy-looking creatures punished with the severity proposed by your correspondent.”117
He wrote other letters to the Gardeners’ Chronicle detailing the bees’ behavior and the fertilization of beans,118 weighing in on an ongoing debate over to what extent crossing (by insects) was necessary for bean fertilization, while crowdsourcing by appealing to readers for observations from their own gardens. A Hampshire gardener named Henry Coe even did an experiment for him, planting twelve lots of bean plants each distinctive in seed color, and carefully collecting the beans eventually produced by each. Analyzing samples sent by Coe, Darwin marveled at the variation: “Beans of new colours have appeared, such as pure white, bright purple, yellow, and many are much mottled. Not one of the twelve lots has transmitted its own tint to all the Beans produced by it; nevertheless, the dark Beans have clearly produced a greater number of dark, and the light coloured Beans a greater number of light colour. The mottling seems to have been strongly inherited, but always increased.”119 The explanation to Darwin was clear: though beans self-pollinate pretty well, insects really mix things up.
Every one who has looked at the flower of the Kidney Bean must have noticed in how curious a manner the pistil with its tubular keel-pistil curls like a French horn to the left side—the flower being viewed in front. Bees, owing to the greater ease with which they can reach the copious nectar from the left side, invariably stand on the left wing-petal; their weight and the effort of sucking depresses this petal, which, for its attachment to the keel-petal, causes the pistil to protrude. On the pistil beneath the stigma there is a brush of fine hairs, which when the pistil is moved backwards and forwards, sweeps the pollen already shed out of the tubular and curled keel-petal, and gradually pushes it on to the stigma. I have repeatedly tried this by gently moving the wing petals of a lately expanded flower. Hence the movement of the pistil indirectly caused by the bees would appear to aid in the fertilisation of the flower by its own pollen; but besides this, pollen from the other flowers of the Kidney Bean sometimes adheres to the right side of the head and body of the bees, and this can scarcely fail occasionally to be left on the humid stigma, quite close to which, on the left side, the bees invariably insert their proboscis. Believing that the brush on the pistil, its backward and forward curling movement, its protrusion on the left side, and the constant alighting of the bees on the same side, were not accidental coincidences, but were connected with, perhaps necessary to, the fertilisation of the flower, I examined the flowers just before their expansion. The pollen is then already shed; but from its position just beneath the stigma, and from its coherence, I doubt whether it could get on the stigma, without some movement of the wing petals; and I further doubt whether any movement, which the wind might cause, would suffice. I may add that all which I have here described occurs in a lesser degree with Lathyrus grandiflorus. To test the agency of the bees, I put on three occasions a few flowers within bottles and under gauze: half of these I left quite undisturbed; of the other half I daily moved the left wing-petal, exactly as a bee would have done whilst sucking. Not one of the undisturbed flowers set a pod, whereas the greater number (but not all) of those which I moved, and which were treated in no other respect differently, set fine pods with good seeds. I am aware that this little experiment ought to have been repeated many times; and I may be greatly mistaken, but my belief at present is, that if every bee in Britain were destroyed, we should not again see a pod on our Kidney Beans. These facts make me curious to know the meaning of Mr. Swayne’s allusion to the good arising from the artificial fertilisation of early Beans. I am also astonished that the varieties of the Kidney Bean can be raised true when grown near each other. I should have expected that they would have been crossed by the bees bringing pollen from other varieties; and I should be infinitely obliged for any information on this head from any of your correspondents. As I have mentioned bees, a little fact which surprised me may be worth giving:—One day I saw for the first time several large humble-bees visiting my rows of the tall scarlet Kidney Bean; they were not sucking at the mouth of the flower, but cutting holes through the calyx, and thus extracting the nectar. I watched this with some attention, for though it is a common thing in many kinds of flowers to see humble-bees sucking through a hole already made, I have not very often seen them in the act of cutting. As these humble-bees had to cut a hole in almost every flower, it was clear that this was the first day on which they had visited my Kidney Beans. I had previously watched every day for some weeks, and often several times daily, the hive-bees, and had seen them always sucking at the mouth of the flower. And here comes the curious point: the very next day after the humble-bees had cut the holes, every single hive bee, without exception, instead of alighting on the left wing-petal, flew straight to the calyx and sucked through the cut hole; and so they continued to do for many following days. Now how did the hive-bees find out that the holes had been made? Instinct seems to be here out of the question, as the Kidney Bean is an exotic. The holes could scarcely be seen from any point, and not at all from the mouth of the flower, where the hive-bees hitherto had invariably alighted. I doubt whether they were guided by a stronger odour of the nectar escaping through the cut holes; for I have found in the case of the little blue Lobelia, which is a prime favourite of the hive-bee, that cutting off the lower striped petals deceived them; they seem to think the mutilated flowers are withered, and they pass them over unnoticed. Hence I am strongly inclined to believe that the hive-bees saw the humble-bees at work, and well understanding what they were at, rationally took immediate advantage of the shorter path thus made to the nectar.
Beans were called into service in a variety of Darwin’s other studies. He reported growth experiments with crossed and selfed plants in Cross and Self Fertilisation, discussed their twining abilities in Climbing Plants, and, in Movement, documented the “sleep” movement of bean leaves and the circular circumnutation of both upward-growing cotyledons and downward-growing radicles, comparing the latter to “a burrowing animal such as a mole” seeking to penetrate the ground perpendicularly, moving from side to side to avoid stones. Indeed, beans were as much a research staple as kitchen staple for Darwin.
Pinguicula vulgaris. Water and bodycolor on vellum by Georg Dionysius Ehret, in Flowers, Moths, Butterflies and Shells.