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Room at the Top
On the movements and habits of climbing plants (1865)
Why was Darwin interested in climbing plants? There's the connection to natural selection, of course, and the gradual modification of leaves (and other structures) into climbing aids. Indeed as we will see, Darwin thought that when it came to the gradual transformation of a structure evolved for one purpose into one modified for something quite different, vines offered about the best example anyone could wish for. But to a large extent it was the barnacle story all over again; he got started and found he couldn't stop. The initial spur, in 1858, was a short paper by American botanist Asa Gray on the tendrils of a plant in the marrow family. Darwin grew some seeds Gray sent him, and was immediately 'fascinated and perplexed' by the movements of the tendrils; they looked complex, but he suspected that some simple principles were at work. And that was it, he was hooked: 'I procured various other kinds of climbing plants, and studied the whole subject.' And when Darwin says 'the whole subject', he really meant 'the whole subject'.
Darwin had already been working for years on pollination of orchids, and his reaction to some of the mechanisms of pollen transfer in that great group of plants was that 'I never saw anything so beautiful'. But he went on to conclude that 'Some of the adaptations displayed by climbing plants are as beautiful as those by orchids for ensuring cross-fertilisation.' And for Darwin, that beauty is the key; any product of natural selection, performing a vital function with economy and elegance, was a thing of beauty.
I can only agree. Climbing plants are beautiful, and wonderful – and addictive too, as Darwin discovered.
Right at the start of The movements and habits of climbing plants, Darwin observes that climbing plants come in various sorts, and that twining is 'the largest subdivision, and is apparently the primordial and simplest condition of the class'. He reasoned that if you wanted to turn a non-climbing plant into a climber, the simplest way to do that would be to have it twine; all the other options require more radical modifications. He next observed that in the hop, a typical climber, the young shoot 'may be seen to bend to one side and to travel slowly round towards all points of the compass, moving, like the hands of a watch, with the sun'.
His reaction to this observation shows just what a careful scientist Darwin was. For of course, the question is: how does this movement occur? Contemporaries of Darwin tended to assume that the stem itself twisted. It's easy to believe this because, as Darwin noted, 'the axes of nearly all twining plants are really twisted'. But he also noted that they weren't twisted enough; a stem may have undergone thirty or more revolutions, but show evidence of only two or three twists. Ergo, the twisting and the revolving of the shoot are not related. In fact the twists are the simple mechanical outcome of wrapping a stem around a support, but the revolutions are caused by a zone of growth moving around the stem, alternately pushing the growing tip over to one side. Thus the revolving movement is more accurately described, as Darwin put it, as a 'continuous bowing movement directed successively to all points of the compass'.
The purpose of this movement, of course, is to increase the chances of bumping into a support. But more than that, it holds the key to twining itself. As usual, some of Darwin's contemporaries had the wrong idea, assuming that the stems of twining climbers must be sensitive to touch (in Darwin's word, irritable), so that they actually bend towards any object they touch. But Darwin couldn't persuade twining stems to respond to anything, and concluded that twining round a support was just a natural extension of the normal revolving movement. Or, as he put it:
If a man swings a rope round his head, and the end hits a stick, it will coil round the stick according to the direction of the swinging movement; so it is with a twining plant, a line of growth travelling round the free part of the shoot causing it to bend towards the opposite side, and this replaces the momentum of the free end of the rope.
This worked so well that Darwin concluded that irritability was unnecessary, a sentiment he expressed in classic Darwinian style:
I conclude that twining stems are not irritable; and indeed it is not probable that they should be so, as nature always economizes her means, and irritability would have been superfluous.
Which also explains something important to gardeners. If a twining stem really did hug a potential support, in the hope that it might eventually travel right round and come back to where it started, it might by such means manage to hang on to even a very thick support. But, Darwin found, this was not so:
I placed some long revolving shoots of a Wistaria [sic] close to a post of between 5 and 6 inches [13-15 cm] in diameter, but, though aided by me in many ways, they could not wind round it. This apparently was due to the flexure of the shoot, whilst winding round an object so gently curved as this post, not being sufficient to hold the shoot to its place when the growing surface crept round to the opposite surface of the shoot; so that it was withdrawn at each revolution from its support.
In other words, the normal revolving movement is enough to cause a stem to wrap itself round a support, provided that support is relatively narrow; but beyond a certain diameter, although the moving zone of growth starts out by pushing the stem towards the support, it soon pushes it away again, and no twining is achieved.
So Darwin had shown, to his own satisfaction, that twiners were unable to attach to thick supports, something that many gardeners will confirm from their own experience. But he was uneasily aware that he had been able to experiment only on temperate climbers, and that the tropics were full of climbers, some of them enormous. Could they do things that temperate climbers couldn't?
To find out, Darwin did what he usually did when he had a botanical problem; in November 1864 he wrote to Hooker at Kew:
Answer this only if by chance you can so surely that I may give it on your authority. — Can any spirally twining plant (not having tendrils) twine round a tree or post one foot or upwards in diameter? Our temperate climbers cannot, from a peculiarity in their movements, twine round a post even six inches in diameter. I suspect some of the Tropical Twiners can manage a much greater diameter. — Are there thick columns in the Houses at Kew?
Hooker replied with a mixture of his own observations and those reported to him by others, but Darwin was suspicious of third-hand data, and Hooker's observations contained an important caveat: 'We have columns of 6 in. in our houses & have climbers on some, but these have been helped, — whether necessarily or no I will not say.'
That 'help' is crucial; with enough coercion, twining climbers can be made to perform all kinds of tricks. I've seen both our commonly cultivated species of wisteria wound round thick columns at West Dean Gardens in Sussex, but they had plainly been strongly 'encouraged' to do so; one clear sign of this was that both were (by chance) growing in the wrong direction (see next section). So although Darwin reported Hooker's observations in Climbing Plants, he was inclined to trust his own experience, which told him that neither wisteria, nor honeysuckle, nor several other species could be persuaded of their own volition to climb up anything more than about 11-12 cm in diameter.
The inability of twiners to grasp wide supports, even in the tropics, has been confirmed by more recent research. In one study in Ecuador, lianas were found growing on trees up to a metre in diameter, but the largest trees supported only root climbers, which are not limited by host diameter. It was also obvious that twiners on wide hosts were relatively thick themselves, indicating that they had started out on thinner hosts and that the two had grown together. If we look only at thin twiners (less than 1 cm diameter), which had presumably colonised their hosts relatively recently, 90 per cent grew on hosts of less than 8 cm diameter. Twiners may also get into the crowns of big trees by using smaller trees, or even other climbers, as 'ladders'.
Twining left and right
So twining stems revolve and, once they hit a support, this revolving leads naturally to twining in whichever direction they had previously been revolving. But which direction is that? Darwin carefully observed numerous twiners, classifying them as twining 'with the sun' or 'against the sun'. There's nothing wrong with those labels, but they do mean different things in the northern and southern hemispheres, so it's probably a better idea to have a description of twining direction that works anywhere. A commonly used terminology is clockwise or anticlockwise, but that's even worse. Because now, instead of just knowing which hemisphere you're in, you need to know whether you're on the floor looking up a climbing stem, or up a ladder looking down. Does the earth rotate clockwise or anticlockwise? It depends which pole you're looking at. A better way to describe twining direction is as left-handed or right-handed. In other words, does the twining stem cross the support from lower left to upper right (right-handed), or from lower right to upper left (left-handed). This always gives the same result, irrespective of where you're standing, or which hemisphere you're in.
There are two popular notions about which way plants twine. One, probably the more widespread, is that plants track the apparent daily east-west movement of the sun across the sky. Indeed only the other day, I read a newspaper description of a hop grower carefully helping the young hop stems to twine so that they track the sun (as if they needed any help, or that you could possibly persuade them to do anything else). The other idea is that it's all determined by the Coriolis effect, which is what makes your bathwater drain in different directions (allegedly) in the northern and southern hemispheres. The latter hypothesis predicts right-handed twining in the northern hemisphere and left-handed twining in the southern hemisphere. Predictions of the sun hypothesis are more complicated, but you would still expect the direction of twining to vary with hemisphere and latitude.
In 2007, New Zealand ecologist Angela Moles published a paper showing that about 92 per cent of the world's twining plants twine in a right-handed helix, and this is true everywhere on the planet, so both hypotheses are wrong, and hops are in the small left-handed minority. But Darwin's observations, 140 years earlier, already strongly hinted at a preponderance of right-handed climbers; of the 40 species he studied, 27 were right-handed and 13 left-handed (if the appropriate statistical test had been invented at the time, this difference would have proved to be significant: i.e. the proportions of left and right-handed twiners was not random). In fact Darwin seems to suggest that this was old news even then: 'A greater number of twiners revolve in a course opposed to that of the sun, or to the hands of a watch, than in the reversed course, and, consequently, the majority, as is well known, ascend their supports from left to right' (my italics).
Curiously, Darwin noted 'I have seen no instance of two species of the same genus twining in opposite directions, and such cases must be rare'. One of the species he describes is Wisteria sinensis, which he correctly notes is right-handed. The Chinese wisteria was introduced to Britain in 1816, and by 1835 was widely available, so it's not surprising that Darwin knew it. Down House, Darwin's home in Kent, is today home to a large Chinese wisteria, and the romantic in me would like to believe that it was planted by Darwin himself. Unfortunately, although old watercolours and black and white photographs show Down House covered by climbers, these are all long gone, and we don't know what they were. A photograph from as recently as 1994 shows Down House completely devoid of climbers, so its present covering must all date from its acquisition in 1996 by English Heritage, and it may just be an accident that the wisteria is the 'right' one.
It's a pity that Darwin was writing just too soon to be aware of the Japanese wisteria, W. floribunda, which wasn't introduced from Japan (via the Netherlands) until the 1870s, so he was unaware of its left-handed twining (which thus allows it to be separated from its Chinese cousin, even when completely leafless).
Undoubtedly, however, Darwin was right that two species in the same genus twining in opposite directions must be rare; I don't know of any apart from wisteria. Collectors of botanical trivia will be delighted to learn that any hybrid of Japanese wisteria inherits its twining direction from that parent.
The fact that different plants twine in different directions would be interesting even if it were (apparently) random, with half going one way and the other half the other way. But the overwhelming predominance of one direction leads to the obvious question: why? Since twining has clearly evolved independently on many occasions, with very different plants as the raw material, why do we nearly always end up with a right-handed spiral? Surprisingly, this doesn't seem to be a question that bothered Darwin; he simply notes that most twiners are right-handed and leaves it at that.
Or is it surprising? Darwin was intensely curious about almost everything, but he was also always on the lookout for adaptive reasons for why plants and animals had turned out the way they had. Maybe he reasoned that it was hard even to imagine an adaptive explanation for left or right-handedness, and even harder to imagine any way of investigating it. Left-handedness may be rare, but there's no evidence that it's done the plants that have it any harm; both the climbing honeysuckles in my garden seem happy enough, despite their sinister habits. And you can't make a plant change its mind to see what happens – the result of any attempt to do so is a plant that refuses to climb at all.
In fact, even today we don't really know why most plants are right-handed, but nature often is; most people are right-handed, and more than 90 per cent of snail shells coil in right-handed helices. It may simply be that nature is often right-handed at some fundamental molecular level; the DNA molecule is a right-handed helix, although left-handed DNA is theoretically possible.
The basic building materials of life are proteins, themselves constructed from a small palette of amino acids. Amino acids, like most organic molecules (e.g. sugars) can come in two forms, which share exactly the same formula but are mirror images of each other. The two forms are called L and D and, while it's tempting to think of them as left and right-handed, they aren't in any real sense. The interesting thing is that life uses only the L forms of amino acids (no-one really knows why), and when L-amino acids come together to form proteins, they form a right-handed helix, usually called an a–helix. There are proteins with left-handed a–helices, but they are very rare.
It's easy to assume that all this underlying handedness, or chirality, must somehow give rise to the commonly-observed right-handed behaviour of animals and plants. Chinese researchers have tried to trace the twining behaviour of gourd tendrils back to the helical angle of cellulose fibrils at the subcellular level, but I confess that I find their argument hard to follow. For all I know, maybe it goes all the way back to gravitational waves and the Higgs boson or whatever, but if it does then so far no-one has proposed a plausible mechanism. What is abundantly clear, over 140 years later, is that Darwin was probably right to consider the whole question to be one that probably wasn't worth investigating, especially not with the technology available at the time.
As much as Darwin was entertained by twiners, he found that plants that climb using tendrils were even more absorbing. In the first place, tendrils illustrate a crucial feature of natural selection, which is that when faced with some new need, animals and plants rarely evolve some completely new structure or behaviour. Almost always, some existing structure is modified to meet the new requirement. Indeed, the world would be a very different place if living organisms had been designed from scratch, and the endless ways in which old structures are modified for new purposes, often in the most surprising and tortuous ways, is one of the most convincing pieces of evidence that none of them was designed at all. For example, the tiny bones that transfer sound vibrations to the eardrum in mammals started out as part of the reptilian jaw. The tendrils of climbing plants illustrate this principle perfectly. Many tendrils, perhaps the majority, are clearly modified leaves, for example those of the pea. Indeed in some climbers the tendrils are still leaves, but modified to have grasping stalks, as in all the climbing species of clematis.(Continues…)
Excerpted from "Darwin's Most Wonderful Plants"
Copyright © 2018 Ken Thompson.
Excerpted by permission of The University of Chicago Press.
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