Did you know that plants move? I don’t just mean by being blown about in the wind, or sending out creeping roots or runners but actually have parts that move spontaneously as if looking for something?  Plants look static to the naked eye, but all of their parts from tendrils to flowers and leaves move in swaying circles. Of course,  it’s not  that rapid but it is noticeable if you have plenty of time to observe and take careful note –  or you have a slow-motion camera – neither of which  most of us have.

One who did have time was Charles Darwin although only because  he was lying ill in bed in 1863 and had nothing better to do, and of course being Darwin he came up with a theory and then a book on the subject. Of course not everyone took his ideas seriously.

Apologies that the captions are difficult to read:
Left: Suppose the farmer has to keep his crops from coming into the house for shelter?
Centre: We have carnivorous plants but what if by “natural selection” we get man-eating trees? Nice to prune them!
Right: And how dangerous to let children go near voracious Christmas Trees!

The links with the extracts from letters  will take you to the full text at the University of Cambridge’s Darwin Correspondence Project

I began the post with mock cucumbers because  in 1862 the year before his illness Darwin had  read an article in the Proceedings of the American Academy of Arts and Sciences  by his long-time friend Asa Gray, the Harvard  botanist. In  “Note on the coiling of tendrils”  Gray described how he had discovered that plant tendrils were sensitive to touch.

That intrigued Darwin and so he wrote to Gray saying he’d like to try the experiments himself and asking “what would be good & easy plant to raise in pot”. Seeds of mock cucumber (Echinocystis lobata) duly arrived. They were planted and fortunately were growing nicely when Darwin was confined to bed and had time to observe them closely.

For more on the friendship and collaboration between Gray and Darwin see Janet Browne,  “Asa Gray and Charles Darwin: Corresponding naturalists.” Harvard Papers in Botany 2010

Darwin wrote to another long-standing friend Joseph Hooker, the Director of Kew:  “I am getting very much amused by my tendrils— it is just the sort of niggling work which suits me & takes up no time & rather rests me whilst writing. So will you just think whether you know any plant, which you could give or lend me or I could buy with tendrils remarkable in any way, for development, for odd or peculiar structure or even for odd place in natural arrangement. I have seen or can see Cucurbitaceæ—Passion-flowers—Virginian creeper—Cissus discolor—common Pea & Everlasting pea.”   Hooker replied : Your observations on Tendrils &c are most curious & novel, & I am delighted that you are going on with them— you are “facile princeps” of observers.  I am looking out some climbers that may serve your purpose & hope to send you Bignonia, Cobæa, Gloriosa, Flagellaria & perhaps others.” Other plants came via James Veitch,  the country’s leading nurseryman at the Royal Exotic Nursery in Chelsea,  whom Darwin thanked “for having generously given me a large collection of fine specimens of climbing plants.”

Soon every one of his windowsills must have been covered with plants all growing under his watchful eye. Luckily he’d also had a  hothouse  built in 1862 which  provided even more experimental space.  Many further letters about the subject were written soon afterwards to both Gray and Hooker, but also to Daniel Oliver who worked at Kew and was Professor Botany at University College London. These can be found at the Darwin Correspondence Project

Four months later Darwin was sufficiently recovered to sit on a chair outside where he watched the shoots of hops seeking out suitable supports to take hold of and climb. He discovered that although “the two or three first-formed internodes are straight and remain stationary… the next-formed, whilst very young, 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.” The process took  just over  2 hours and “the revolving movement continues as long as the plant continues to grow.”

There a short [I minute 30 secs]  YouTube videoclip showing hope shoots doing this 

By early spring of 1864 he told Gray that he was  “able most days now to get to my Hot-house   I amuse myself a little by looking at climbing plants.”

But problems and questions kept cropping up. He corresponded with Gray, Hooker and Oliver continuously. Were tendrils modified leaves? Modified stems? Or perhaps some form of  modified flower?   ‘By the way I observed yesterday an odd little fact, that in the vine the Flower buds are borne on a true tendril, for the whole mass of flowers steadily revolves‘.

He tried growing and watching even more plants including a tropical pitcher plant (Nepenthes) which he grew  in his hothouse. Kew of course had better and bigger specimens so  he asked Oliver to “observe this one point.- whether a leaf with a pitcher has ever fairly & closely wound round a stick or support”. It was Hooker who responded  that, “Nepenthes climbs famously by the stalk of the pitcher & the pitcher fully develops after the process-it also thickens”.

He was urged by Hooker to write it all up but the proposed paper got longer and longer and he had more things he wanted to check and test. Darwin now devised experiments to monitor what his plants were doing, even when he wasn’t looking.

He realised that they were doing something complex, with tendrils and sometimes stems spontaneously revolving looking for support. He called the phenomenon  circumnutation.  He was delighted telling his son  “My hobby-horse at present is Tendrils; they are more sensitive to a touch than your finger; & wonderfully crafty & sagacious”. He believed he had dispelled the mistaken old view ‘that animals moved & plants did not‘.  Later in  his autobiography he noted that “some of the adaptations displayed by climbing plants are as beautiful as those by Orchids for ensuring cross-fertilisation.”

 

However after reporting his findings to Gray he got a bit of shock. Gray told him that quite a lot of what he thought was new was actually already known. Darwin replied ” I am rather taken aback …I can find nothing in any book which I have: neither Hooker nor Oliver knew anything of these movements…. Pray tell me whether anything has been published on this subject?”   In reply Gray asked “What are Hooker & Oliver (the latter a Professor too) about, and where have they lived not to know anything of them?” and then pointing out two sources, before adding “do not abandon this subject, for it will be fruitful in your hands.”

The next problem was that although he could observe what the tendrils were doing he did not understand  how it happened.  Just how crafty and sagacious were these plants?  How could he record and understand these movements in a way which might reveal what was going on?  Being Darwin he came up with an ingenious method.

He placed a plant between a sheet of paper and a glass plate and marked a reference point on the paper, then attached a thin wire to a leaf or bud. At regular intervals he was able to record the way the plants had moved in relation to the reference point by marking its position on the glass plate. The resultant marks could be joined up to reveal the track the plant had taken. The results are weirdly angular and taken out of context probably appear meaningless – indeed as Nathalie Lawrence observed  “it would be easy to think that they might be the tracks of a small animal — a woodlouse, beetle, or perhaps a mouse with a short attention span. They seem like the staccato perambulations of a creature that does not have a clear purpose, rambling across the paper. But that is because these are static, two-dimensional renderings of movements that occurred in three dimensions… In the days before time-lapse photography and cinematography, this was an incredibly creative way of capturing plant movement to make it meaningful for humans.”

Not content with just a few measurements like this Darwin went on to trace the tracks made by hundreds of plants, and began to build a picture of their different  movement patterns. He became convinced that like animals, plants were exhibiting deliberate behaviour. Of course there were differences. Animal movements are easy to spot whereas plants move primarily because they were trying to grow. Remember this is less than 4 years after his publication of Origin of Species and so  he also began to wonder if there  might be links to his evolutionary theories.  Were plants responding to their environments, and developing different ways of growing in order to survive, develop and reproduce more successfully.  It took him two more years to produce his first long paper on the subject: On the Movements and Habits of Climbing Plants. which was read at the Linnean Society and then published in their Journal in 1865,. In it he spelled out in detail his experiments and their results.

Darwin himself thought the paper a little dense and disordered and  revised it for publication ten years later but there was positive reaction anyway.  He received a letter from Benjamin Walsh, with whom he had been at Cambridge who wrote  telling him that “this discovery of their [ie plants] sweeping circles & groping in the dark for support, like a blind Cyclops, is very astonishing. And yet, when we consider that plants have no eyes, it is evidently the only possible way, & like the other ways of Nature so beautifully simple. The gradations too that you trace between the different kinds of Climbers are of high philosophical interest”.  Charles Kingsley the clergyman and novelist also wrote telling Darwin he had read “with delight & instruction your paper on Climbing plants” and wished he “could begin to study Nature anew, now that you have made it to me a live thing; not a dead collection of names,” and there were positive comments and suggestions from abroad as well.