We are utterly dependent on plants. We wake up in houses made of wood from the forests of Maine, pour a cup of coffee brewed from coffee beans grown in Brazil, throw on a T-shirt made of Egyptian cotton, print out a report on paper, and drive our kids to school in cars with tires made of rubber that was grown in Africa and fueled by gasoline derived from cycads that died millions of years ago…. And plants continue to inspire and amaze us: the mighty sequoias are the largest singular, independent organisms on earth, algae are some of the smallest, and roses definitely make anyone smile.
Knowing what plants do for us, why not take a moment to find out more about what scientists have found out about them ?
I’ve lived a relatively plant-oblivious life – until about six months ago. And now I’ll talk about my plants (not even interesting ones, basic beginner stuff) like old ladies talk about their bunions.
It’s all @drunkphyto’s fault.
I was minding my own business when someone retweeted her tweet into my feed last September: “The smell of cut grass is the grass releasing a wounding compound into the air to warn other plants that they were injured. You are smelling their screams.” I immediately thought of Seth Fried’s “Animacula”, a short story in the form of a lab report about organisms with strange properties, including screaming. Oh, and Liz Ziemska’s “The Mushroom Queen” which acquainted me with the interconnectedness of fungi via mycelia.
I emailed @DrunkPhyto to tell her how excited I was about all this (yeah, I know) and, to my surprise, she gave me a friendly reply rather than a restraining order. She recommended a number of books, one of which was Chamovitz. So it ended up on my reading list. And I started eyeing the plant stand in the supermarket, until I finally brought home a tiny philodendron, then an ivy, and an oxalis, and various flowers….
I was in for another surprise. As I started reading, I realized I’d taken all these moocs on biology, physiology, biochem, anatomy, and other sciency topics, and while I’d encountered cell respiration and the Michaelis-Menten equation multiple times, I’d never learned anything specific to plant biology. I didn’t even know how photosynthesis worked! So I checked edX for any moocs on plant bio, and found little beyond agricultural ecology. Ah, but on Coursera, I found… Understanding Plants: What a Plant Knows , taught by Daniel Chamovitz! So of course I signed up. It follows the book very closely, and includes very helpful diagrams the book lacks. Double bonus: He has a second course, Understanding Plants: Fundamentals of Plant Biology , which I will take as soon as I finish up the biochem I’m struggling with.
How way leads on to way…
Plants must be aware of the dynamic visual environment around them in order to survive. They need to know the direction, amount, duration, and color of light to do so. ….Plants don’t have a nervous system that translates light signals into pictures. Instead, they translate light signals into different cues for growth. Plants don’t have eyes, just as we don’t have leaves.
But we can both detect light.
The book’s approach is to examine how plants sense their environment, through chapters like What a Plant Sees, What a Plant Feels, How a Plant Knows Where It Is, What a Plant Remembers. For each sense, the approach is to look at the human equivalent – say, sight – and break it down to its fundamental quality – sensing light – while pointing out key differences between the human version and the plant version – plants don’t have brains to interpret light signals into pictures – and presenting experimental evidence and theories for ecological significance of the sense.
There’s a fair amount of technical detail for a general readership book. The basics of electrochemical conduction, for example, and the regulation of water through ion transport to cause movement; gene expression and epigenetics; receptors and phytochromes. The experiments that revealed various processes and qualities are described in detail. I have to admit, I was surprised that Darwin was such a plant buff, proving that plants sense light in the tips of shoots. One of the most ingenious experiments was by Thomas Andrew Knight, a 19th century gentleman (rather than a scientist) who concocted a kind of water wheel to create centrifugal force to understand the role of gravity in plant growth, the International Space Shuttle being a couple of centuries in the future.
One of the most interesting chapters was What a Plant Hears, for several reasons. Caution: Spoiler ahead! First, it was a negative finding, and, as Chamovitz points out in his mooc, “one of the other problems in scientific research is that you can’t publish negative results.” This is particularly pertinent to this chapter, since a poorly-designed study in the 60s, coupled with a pop-science (in the worst sense of the phrase) book, had everyone convinced that plants like to be talked to, and they prefer classical music to rock. I’ll admit, I thought this was the case until I read this chapter; I had no idea the study was flawed and the hypotheses invalid. But because no one wants to publish negative results, failures to replicate the study weren’t anywhere near as publicized as the original work.
Even more interesting, the mooc contains a post-production video updating the hearing lecture, since later experiments have shown that plants do show responses to low frequency sounds, possibly via touch sensors (which is, fundamentally, what hearing is), and this may be related to sending roots in the direction of water. As Chamovitz says, “Science is a self-correcting system,” and new research leads to new theories.
Our dictionary’s definition of smell excludes plants from discussion. They are removed from our traditional understandings of the olfactory world because they do not have a nervous system, and olfaction for a plant is obviously a nose-less process. But let’s say we tweak this definition to “the ability to perceive odor or scent through stimuli.” Plants are indeed more than remedial smellers. What odors does a plant perceive, and how do smells influence a plant’s behavior?
The chapter on smell was also particularly interesting. Just like us, plants have receptors for volatile chemical molecules, which are the basis of smell. Anyone who has sped up the ripening of a peach or avocado by placing it in a paper bag with a ripe banana has used this sense: ethylene is given off by ripe fruits and signals other fruits to ripen. I learned this practice goes back many centuries, though it used other means: incense in China, for example.
And here’s where the book’s approach really works for me: given that this is the case, why would this happen? What’s the evolutionary advantage to having one ripe peach encourage others to ripen as well?
From an ecological perspective, this has an advantage in ensuring seed dispersal as well. Animals are attracted to ready-to-eat fruits like peaches and berries. A full display of soft fruits brought on by the ethylene-induced wave guarantees an easily identifiable market for animals, which then disperse the seeds as they go about their daily business.
So it isn’t that peach trees thought it would be a good idea if they did this; it’s that those plants that had this facility, however it was acquired (by mutation?) would have better reproductive success than those that didn’t. This is evolution in a nutshell. This is also my own musing, not a point made explicitly in the book, so if I’m off-base, tell me.
It’s this sense of smell that @DrunkPhyto was (slyly) referring to with “smelling their screams”. This exact point comes up when considering that an injured leaf will release a volatile chemical, and other leaves, on the plant and on other plants, will respond to it with self-protective measures:
While the phenomenon of plants being influenced by their neighbors through airborne chemical signals is now an accepted scientific paradigm, the question remains: are plants truly communicating with each other (in other words, purposely warning each other of approaching danger), or are the healthy ones just eavesdropping on a soliloquy by the infested plants, which do not intend to be heard?
There’s no real answer to this question, but again resorting to evolutionary advantage, plants that warn their own leaves to defend against intruders would likely survive more than plants that didn’t. How the “altruism” of warning other plants comes into it is murkier, though it’s scientifically doubted.
We don’t typically think of memory in connection with plants, but it turns out we can. Again, Chamovitz breaks down memory into its essential parts – storage, encoding, and retrieval – and shows how this works in an organism with no brain, no hippocampus. The Venus Flytrap serves as an excellent example of short-term memory: about 20 seconds. Plants that want to bloom or seed at specific times of the year keep track of the length of the day via genetic suppression or expression; this serves as a kind of medium-range memory. And the most interesting memory of all, long-term memory, spans generations via epigenetics, a topic I know far too little about:
…Not only do the stressed plants make new combinations of DNA but their offspring also make the new combinations, even though they themselves had never been directly exposed to any stress. The stress in the parents caused a stable heritable change that was passed on to all their offspring: the plants behaved as if they had been stressed.… In other words, stressed parents give rise to offspring that grew better under harsh conditions compared with regular plants.
Human experience tells a different story, since human offspring are subjected to other inputs beyond genetic inheritance. But it’s an amazing paragraph: what doesn’t kill a plant, makes the species stronger.
A look at awareness – consciousness – ends the book; it’s not as far-fetched as you might think. I myself hold two conflicting instincts about this sort of thing. I’ve always found it impossible to understand how a plant could “know” it’s time to bloom or seed, or for that matter how a red blood cell knows to pick up oxygen in the lungs and drop it off in the tissues. The biochem mooc I’m taking just did a wonderful lesson on that process, in fact, and it helped to clarify that it’s all about osmosis, competing pressures, and electrical charges repelling and attracting each other. But you could say the same thing about our brains: maybe all the art, belief, and knowledge is just a matter of manipulating matter and energy, no matter how much it feels like we control it with our will. On the other hand, I find it troubling when anyone declares some ethereal quality – like art, or religion, or emotion – is what makes people special, and when it turns out bees dance and whales communicate, the goalposts get moved to keep humans unique. I don’t try to reconcile these two ideas. Like Whitman, very well, I contradict myself; I am large, I contain multitudes.
Granted I have little to compare it to, but I don’t think I could have picked a better entrée to plant biology than this book. It combines a hint of romanticism with solid scientific evidence, and bounces off my prior learning (if unorthodox, via moocs and youtube) in biology and neuroscience to bridge the gap between human and botanical. Finding a mooc attached to it was a super-deluxe Easter egg.
For readers who’d rather not bother with the technical details, there’s still plenty to enjoy. And who knows, you might just come away with curiosity about something you always thought was way over there somewhere. Way does lead on to way, after all.