Holiday season behind us, I walk down the street. Christmas trees are strewn across the pavements for collection. Well, that is the way we talk about it, at least. Their root systems lobbed off doesn’t seem to bother holiday merrymakers, perhaps because that part of the tree is invisible anyway. But roots are complex structures comprising a significant quantity of a tree’s mass and volume. And so, it must be asked: in what sense do we really decorate ‘trees’? Soaked in water, the tree continues to perform in minimal ways we think make it a tree; it sits there, stays green for a while, and emits fragrance from its resins. But like believing a corpse is merely sleeping because his nails and hair are still growing, are we oblivious to a macabre spectacle? What is lost when roots are cut off?
In his last decades, Charles Darwin was increasingly devoted to studying plants. He wrote a number of illuminating but less well-known books on flowers, plant evolution and behaviour. Co-written with his son, On the power of movement in plants (1880) was his penultimate study. Its last few pages propose an arresting hypothesis that laid largely buried for over a hundred years. After conducting several experiments— pressing or burning root tip apices and examining subsequent changes to plant growth —they noticed an interesting phenomenon. If burnt on one side of a root tip, the plant’s aerial parts would grow the other way, even though this response would not occur were it burnt anywhere else (including further up the root). Injured plants seem to respond as a whole to local impacts on individual root tips. The root tips, they surmised, therefore play a special role in picking up relevant information and centralising a coordinated whole-organism response to it. The Darwins concluded root apices functioned analogously to a simple brain.
Is it absurd to use neural analogies to understand plants? Some assert it is plainly so (e.g., Alpi et al., 2007). But many metaphors used to describe neurons and their synapses were themselves borrowed from botany. Consider ‘arborisation’, ‘dendrite branching’ (double whammy there), and neural ‘pruning’: if plants prove an effective source to describe aspects of neurons, why deem it anthropomorphic (or animal-centric) to go the other way and investigate how neural thinking might better help us understand plants?
The Darwins’ intriguing idea remained uprooted until the rise of contemporary plant behaviour and signalling research (Baluska et al, 2009). According to these authors, plants are analogous to animals with their heads buried in the soil. Superficially, this seems to make sense— at least according to our mental image of the typical animal and the typical plant —roots, like mouths and nostrils, are where plants take in nutrients and gases from the air, while leaves and flowers are excretory and sexual organs respectively. However, the more important question is not to what extent the upside-down analogy is roughly true, but how much the root system really does coordinate responses to information a plant receives.
One way to approach this question is anatomical. Is the root system organised (or not) ‘like’ a brain? The point is not to find specific similarities. For instance, a chemical that serves as a neurotransmitter in an animal might be doing things broadly served by a different chemical in a plant. On the other hand, that neurotransmitter might exist in plants but be involved in totally unrelated activities. The anatomical approach seeks correlations in structure and function between brains and roots.
This approach immediately leads to a problem. Root system architecture tends to be vertical. Roots break into smaller roots, and so on, without evident channels between them— in obvious contrast to the messy, circular and interconnected nature of neurons in a brain. Lateral connections between parts of the brain are reinforced or atrophy— facilitated, reinforced or softened through use and disuse. It seems intuitive that lateral connections between roots would be a minimum structural requirement for an organ whose function is to coordinate information, because otherwise it would seem hampered by the siloing constraints of its shape. Can something like this be found between a plant’s roots? Perhaps we ought to look at root hairs (and their associated mycelia) as such flexible lateral structures. Like neurons, root hairs are usually long single-celled structures. Their copious
growth means they certainly come into contact with other hairs of their own, or other roots. Root hairs grow and atrophy relatively quickly and easily. Looking at the growth of root hairs might be analogous to dendrite branching, while volatile organic compounds released in the soil regions between root hairs might be roughly synaptic. One concerns transmission along linear tissue, the other across spaces between such tissue. Sadly, research into communicative activity in root hairs is virtually non-existent.
Nevertheless, there is no point in looking for anatomical structures that might be organised like neural networks if no behaviour warrants the search for these structures in the first place. For this reason, a second area of research has to do with plant behaviour. It is certainly the case that coordinated plant responses are well-detailed and commonplace. A lot of plant coordination is owed to the release of hormones, such as jasmonate and auxin. This is not the kind of integrated activity we would be looking for in an organism with something brain-like about it. Instead, we would be looking for a globally coherent activity that involved differentiated responses amongst its parts. For instance, we might look for electric signals transmitted between cells, leading to local but coordinated responses. Electric signalling has been known in plants since even before Darwin’s experiments. Like Darwin’s root apices, its significance was also downplayed until evidence could no longer be ignored (Davies 2006). Action potential, for example, is now recognised as pervasive in plants. More detailed studies into signal transduction in roots, cambium, and other tissue that extends throughout the plant body is needed.
A second issue is that coordinated plant responses do not appear to be as coordinated as, say, those in vertebrates. In investigating plant responses to stimuli, what level of centralising is needed to deem it ‘brain-like control’? Plants may be more decentralised than vertebrates, responding to their worlds more like a confederacy than a dictatorship (Firn, 2004). Response may be either at the cellular level, the tissue level or something more global— depending on the situation. An organism is likely to centralise its response to the extent it needs to, and plants may not need to— or at least not need to as much. But we should be wary of drawing dichotomies across kingdoms. Animal behaviour is not equally centralised across its phylla, either. By any anthropocentric measure, octopuses are highly intelligent— but they have more neurons in their arms than in their heads. On the other hand, citing Shomrat and Levin (2013), mycologist Merlin Sheldrake (2020) points out that flatworms are able to regrow brains once their heads have been cut off, and retain memories of their prior experiences.
When very young, some conifer cuttings can grow new roots, but not once the tree is big enough to wrap with tinsel and adorn with red balls. It would seem only small and simple bodies can get by without brains— or roots —long enough to sprout fresh ones. With or without an artificial supply of nutrients, such trees slowly die. Whatever it is, something more fundamental than a flatworm’s brain was taken from these firs and pines, their colours dull and bodies brittle, awaiting pick-up above pools of dry dead needles.
References
Alpi, A. et al. (2007) ‘Plant neurobiology: No brain, no gain?’ TRENDS in Plant Science 12 (4): 135-136
Baluska, F.; Mancuso, S.; Volkmann, D. & Barlow, P. W. (2009) ‘The “root-brain” hypothesis of Charles and Francis Darwin: Revival after more than 125 years.’ Plant Signaling & Behavior, 4(12): 1121–1127
Darwin, C and Darwin, F. (1880) On the power of movement in plants. John Murray: Edinburgh
Davies, E. (2006) ‘Electrical Signals in Plants: Facts and Hypotheses,’ in Volkov A.G. (ed.) Plant Electrophysiology. Springer: Berlin, Heidelberg.
Firn, R. (2004) ‘Plant intelligence: an alternative point of view,’ in Annals of Botany, 93(4): 345–351
Sheldrake, M. (2020) Entangled Life. The Bodley Head: London
Shomrat, T. & Levin, M. (2013) ‘An automated training paradigm reveals long-term memory in planarians and its persistence through head regeneration,’ in The Journal of Experimental Biology, 216(20): 3799 LP – 3810
Trewawas, A. (2015) Plant behaviour and intelligence. Oxford University Press: Oxford, UK