PLANT MOVEMENT
Arachis is a genus with nearly seventy species, growing in South America’s dry tropical and subtropical grasslands. Only one is cultivated agriculturally, the common peanut Arachis hypogaea, a species of hybrid origin that is thought to have arisen several millennia ago in the Andean region. Peanuts are now grown commercially in warm temperate areas around the world, a major human food source but also animal fodder and ground cover.
The peanut genus is derived from the Greek word for vetch, arakos, also a legume, while the species epithet hypogaea, “beneath the earth,” refers to the curious habit of geocarpy, fruit development underground—perhaps originally a nifty adaptation for both safeguarding seeds from predation and automatically planting them. The yellow flowers bear what appears to be an ordinary stalk, or peduncle, but is actually a long calyx tube (hypanthium), at the base of which is the ovary. Peanut flowers mainly self-pollinate, a common feature in many legumes, and the pollen tubes successfully make their way down to the basal ovary. They then produce an elongated “peg” (the “gynophore” in Darwin’s terms), a stalk-like structure that extends from beneath the ovary, bearing the developing seed pod down into the soil where the fruits develop. With the help of William Thiselton-Dyer, assistant director of the Royal Botanic Gardens, Kew, Darwin procured potted peanut plants for study. In the course of investigating what he called geotropism and apogeotropism—plant movement toward or away from the earth, respectively, in response to gravity—he traced their movement on a vertically held pane of glass, finding that the growing pegs circumnutate, slowly rotating in an elliptical path as they grow downward.
Arachis hypogaea—The shape of a leaf, with its two pairs of leaflets, is shown at A; and a leaf asleep, traced from a photograph (made by the aid of aluminium light), is given at B. The two terminal leaflets twist round at night until their blades stand vertically, and approach each other until they meet, at the same time moving a little upwards and backwards. The two lateral leaflets meet each other in this same manner, but move to a greater extent forwards, that is, in a contrary direction to the two terminal leaflets, which they partially embrace. Thus all four leaflets form together a single packet, with their edges directed to the zenith, and with their lower surfaces turned outwards. … The petioles are inclined upwards during the day, but sink at night, so as to stand at about right angles with the stem. The amount of sinking was measured only on one occasion and found to be 39°. A petiole was secured to a stick at the base of the two terminal leaflets, and the circumnutating movement of one of these leaflets was traced from 6.40 A.M. to 10.40 P.M., the plant being illuminated from above. … During the 16 h. the leaflet moved thrice up and thrice down, and as the ascending and descending lines did not coincide, three ellipses were formed. …
Arachis hypogaea: A, leaf during the day, seen from vertically above; B, leaf asleep, seen laterally; copied from a photograph. Figures much reduced.
The flowers, which bury themselves, rise from stiff branches a few inches above the ground, and stand upright. After they have fallen off, the gynophore, that is the part which supports the ovarium, grows to a great length, even to 3 or 4 inches, and bends perpendicularly downwards. It resembles closely a peduncle, but has a smooth and pointed apex, which contains the ovules, and is at first not in the least enlarged. The apex after reaching the ground penetrates it, in one case observed by us to a depth of 1 inch, and in another to 0.7 inch. It there becomes developed into a large pod. Flowers which are seated too high on the plant for the gynophore to reach the ground are said never to produce pods.
The movement of a young gynophore, rather under an inch in length and vertically dependent, was traced during 46 h. by means of a glass filament (with sights) fixed transversely a little above the apex. It plainly circumnutated … whilst increasing in length and growing downwards. It was then raised up, so as to be extended almost horizontally, and the terminal part curved itself downwards, following a nearly straight course during 12 h., but with one attempt to circumnutate, as shown. … After 24 h. it had become nearly vertical. Whether the exciting cause of the downward movement is geotropism or apheliotropism was not ascertained; but probably it is not apheliotropism, as all the gynophores grew straight down towards the ground, whilst the light in the hot-house entered from one side as well as from above. Another and older gynophore, the apex of which had nearly reached the ground, was observed during 3 days in the same manner as the first-mentioned short one; and it was found to be always circumnutating. During the first 34 h. it described a figure which represented four ellipses.
Lastly, a long gynophore, the apex of which had buried itself to the depth of about half an inch, was pulled up and extended horizontally: it quickly began to curve downwards in a zigzag line; but on the following day the terminal bleached portion was a little shrivelled. As the gynophores are rigid and arise from stiff branches, and as they terminate in sharp smooth points, it is probable that they could penetrate the ground by the mere force of growth. But this action must be aided by the circumnutating movement, for fine sand, kept moist, was pressed close round the apex of a gynophore which had reached the ground, and after a few hours it was surrounded by a narrow open crack. After three weeks this gynophore was uncovered, and the apex was found at a depth of rather above half an inch developed into a small, white, oval pod.
Arachis hypogaea: circumnutation of vertically dependent young gynophore, traced on a vertical glass from 10 A.M. July 31st to 8 A.M. Aug. 2nd.
Arachis hypogaea: downward movement of same young gynophore, after being extended horizontally; traced on a vertical glass from 8.30 A.M. to 8.30 P.M. Aug. 2nd.
Darwin was also interested in nyctinasty, the nocturnal “sleep” movement of peanut leaves as they fold their leaves at night. To test the hypothesis that nocturnal leaf-folding was an adaptation to reduce frost damage through radiative heat loss (by avoiding direct exposure of the leaf surface to the night sky), Darwin and his son Francis set out potted peanut plants in the open lawn behind their house on freezing late winter nights, preventing some of the leaves from folding by various means, while letting others move freely. By and large, all the leaves kept from folding were frost-killed or injured, while only about half of the “control” leaves suffered in this way.
Darwin excitedly reported the results to Joseph Hooker at Kew, saying, “I think we have proved that the sleep of plants is to lessen injury to leaves from radiation—This has interested me much and has cost us great labour, as it has been a problem since the time of Linnaeus.”24 The reference here was to a doctoral dissertation, Somnus plantarum (“The Sleep of Plants”), supervised by Linnaeus and published in 1755. Linnaeus thought plants did indeed simply sleep, but Darwin and his son now had evidence of an adaptive explanation for nocturnal leaf movement. Their successes came at a cost, however, as Darwin lamented in the same letter, “we have killed or badly injured a multitude of plants,” urging Hooker to send more as soon as possible. “Unfortunately, there is no time to lose, as there may be few more frosts.”
Bignonia capreolata. Engraved and etched plate by Nicolas Robert, from Denis Dodart, Mémoires pour Servir à l‘Histoire des Plantes.