When it comes to twining plants, Darwin was right
Popular misconceptions about twiners were disproved back in the 19th century, courtesy of Charles Darwin
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, more than 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 lefthanded (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 ascend their supports from left to right”.
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.
Although watercolours and black and white photographs show Down House covered by climbers, these are all long gone. A photograph from 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, Wisteria floribunda, which wasn’t introduced from Japan (via Holland) 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.
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TWIRLING Wisteria floribunda ‘Domino’, main; a wisteria vine, right; and Ipomoea tricolor ‘HeavenlyBlue’, below