Kropotkin's hypothesis was that cooperation between species was as fundamental to life on our planet as natural selection. Although his socialist-motivated ideas have been somewhat downscaled by the evidence of the succeeding century, there are still some truths to be learnt about the mutual aid - or symbiosis if you prefer - between fundamentally different life forms.
I recently read about some experiments in Israel and Germany, which involved such esoteric boffinry as placing laser microphones close to tobacco and tomato plants in order to pick up any ultrasonic noises that they might emit. The plants were heavily pruned or moved into parched soil, in other words, subject to physiological stress.
Analysis of the recordings revealed high-pitch sounds (or in the researchers' words, 'squeals') emanating from their herbaceous guinea pigs. Not only did the sounds vary depending on whether the plant was suffering from mutilation or lack of moisture, but each species (both members of the Solanaceae family) had differing numbers of repetitions and time intervals between each sound. What's even more interesting is the noises differed according to the local invertebrate life, specifically the potential pollinating insects.
In addition to the scientists' equipment, animals such as bats and rodents were placed in the vicinity of the subjects and reacted to the sounds as they were being produced, verifying the shrieks as emanating from the plants. The physiological cause appears to be the movement of air bubbles within liquids such as sap, but how are plants able to perceive the problems, let alone respond to them?
It's been known for some years that plants can communicate with other members of their species via emitting chemical compounds; just think of the odour of freshly cut grass. Forest trees even share nutrients via a symbiotic root system in order to allow smaller members of their species to grow faster - so much for selfish genetics here!
Communication between plants by all three methods, namely direct contact, sound, and chemical odour, suggests purpose and awareness, only without a central nervous system to guide it. This might sound impossible, but then the marine bacteria species Bacillus subtilus uses potassium ions to communicate across its colonies and few would argue that bacterium are more advanced life forms than the kingdom Plantae. We should also remember that in even in animals, brains aren't the be-all and end-all: there are neurons in vertebrate (including human) stomachs and in the arms of cephalopods.
The symbiotic relationship between angiosperms (flowering plants) and pollinating insects evolved in the late Cretaceous, so natural selection has had over sixty-five million years to work on the communications systems between these collaborators. Could it be that plants have evolved a specialist messaging service for their pollinating symbionts, despite having no equivalent of neurons to coordinate it?
Some of the recent Israeli research seems to verify this – and how! When endangered by being cut or deprived of water, the specific noises were not only picked up by pollinating insects, they were acted upon. Insects such as hawk moths flew away from the plants that were suffering drought or mutilation to control specimens on the farthest side of the greenhouse laboratory and laid their eggs upon those plants. Meanwhile, other insects that were known pollinators on the same plant species but not local the region ignored the audio signals. Somehow, there is a level of fine-tuning going on that reveals the sensory world of plants is far superior to what is usually credited.
Parallel experiments successfully tested for the opposite effect. Individual tobacco plants with mature flowers sent messages that attracted the attention of local pollinators such as stilt bugs. All in all, it appears that certain plant species – at least of the Solanaceae family - engage in a form of mutual aid that Kropotkin would be proud of. Not only do plants use ultrasonics to target useful insects, they have developed a messaging service that is regionalised towards those insect species, essentially a dialect rather than a universal language.
While tobacco and tomato plants might not be screaming in pain every time they are cut or lacking water, it seems that they cannot be as easily dismissed as the poorer relation to us animals. The time may be due for a complete reappraisal of their perception capabilities, although amateur researchers would do well to remember that both tomato and tobacco are from the same family as the mandrake and as any Harry Potter fan should know, you wouldn't want to hear those scream!
