Friday, 3 April 2015

so, what does a plant know?

There are a group of genes plants needs to tell light from dark. Crucial to them; but also useful to us. We have them too, taking care of our own light responses. The book (Daniel Chamovitz, What a Plant Knows) opens on this. A hint of inner mechanisms not so dissimilar to our own, but driven by different needs, and without organising intelligence. The title, deliberately provocative, is quickly discarded. The book is not about what a plant knows, because knowing is a frill, an irrelevance. What matters to a plant is stimuli and response, and for this is must have senses.

So, the subject is sensory input and information exchange in plants, and how these influence plant behaviour. There is also, in the careful explanations of laboratory experimentations, hints of tricks and hacks that might work on my own backgarden plants, as well as a suggestion of things plants might prefer you to avoid. Irritating stimulus. Depressing input. Off switches.


Plants use a light receptor called phytochrome to see light, using a sensor in the growing tip. Blue light is used for phototropism, as we know from everyone's favourite primary school biology experiment. Red light is used to set the day length, with a little blast of dusk (infra-red) being used to start the clock, and sunrise (visible red) stopping it. Useful for the plant to know what season it is and for humans seeking to control when a plant flowers. But that's just the simple story; plants have eleven photoreceptors to our five, all keeping the plant in enough (but not too much) light to feed, breed and thrive. So, in addition to facts like plants get jet-lag, and green-algae have a simple eye, I extracted the useful information that night-lights for plants don't have to be overclocked electricity-guzzling grow-lights. Even something small can have a big effect.

I've also found myself saying, here you are, tasty photons! to my plants as I switch on the fairy lights. That's probably not going to have any effect, though.


Existing evidence suggests strongly that plants don't hear, unless the vibrations are big enough to trigger a physical reaction, as in buzz pollination. So any talking to your plants is for your benefit, not theirs; and they'd much rather you kept your hands to themselves, too.


There's a superstar plant called Arabidopsis thalania that's a favourite in the labs. If you touch this plant just a few times a day it will grow slower, squatter and flower later, and if you always touch the same leaf, that leaf with wither and die. Just from being touched a few times a day. If leaves are torn or damaged, chemical and electrical signals (meaning even if a leaf is isolated by a dead stretch of stem, the rest of the plant can still react) prime the rest of the plant that the environment is risky. Touch-me-not indeed. Given how much time we spend chopping, cutting, pinching, pruning and tying, this is both a hard and an easy lesson. Don't fuss your plants; don't cut or even touch anything you don't need to; take it easy. And if you do have to get choppy, expect sulks from neighbouring plants too, because their sense of smell is as good as their sense of touch.


Plants intake air through their stomata, including a bouquet of volatiles produced by themselves and other plants. A few of the famous ones (ethylene, salicylate) are well known. The former is what makes bananas ripen other fruit, and is widely used in the fruit industry. The latter, along with its less well-known cousin methyl jasmonate, is used by plants to warn other parts of itself (and other nearby plants) of damage from disease (methyl salicylate) or pests (methyl jasmonate). I'm pretty sure I went off-book at this point to get a clearer idea of how the pheromonal signalling works and investigate practical garden applications, enough to answer the three obvious questions.
  1. About a metre.
  2. You tearing a leaf will trigger the same responses as pest damage.
  3. Usually a combination of increased leaf toxicity and nectar production to pull in carnivorous insects, but may vary species to species.
Which brings me back to pinching tomato plants, and the thought that it may be useful against loopers. But now we're heading weirdwards. Because plants, like us, have a sixth sense.


That's the sense of where you are, by the way. What's up, what's down. Am I upside down? We do this with otoliths in our ears, tiny stones that sink through fluid under gravity. Plants have statoliths in their roots, which do the same.  Impressive, but unhelpful; I'm not going to be able to alter gravity in my garden, even with the help of a very experienced cat.

More interesting is circumnutation, a regular circular pattern that the growth of plants follows, creating the "thrashing" motion so familiar to us from stop-motion plant films:

Wilhem Pfeffer - Plant Movement by kinetoscope

Thanks to experiments on the International Space Station we now know that circumnutation is intrinsic in plants, but exaggerated by gravity. So leave your seedlings enough space to describe their happy circles. Bear in mind that different plants needs different amounts of space. Sunflowers and beans (which I struggle with every year) need a lot.

Plantmind, plantmotivation, plantmemory

The book ends with a wander through curious incidents of plant memory - delayed responses, signal collection, chattering calcium channels. Morphogenic mysteries, and the useful work of Lysenko (vernalisation). The steady growth from Lamarkism to epigenetics. Vegetal consciousness, such as it is (procedural and non-episodic).

I think of soil and water before I think of light and space, perhaps equating them with food and drink. Critical for me, but just nice extras for the plant. I need to think more about what they want, what they remember, what they think about where they are.

It's a short read, but there are footnotes, links and hints a-plenty to start a fruitful search. Very fruitful! My seeds are germinating better this year, with a little help from my battery powered dinosaur fairy lights.

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