What Arrives
When a caterpillar bites an Arabidopsis leaf, the plant begins producing jasmonate-triggered volatile compounds. Neighboring plants detect these volatiles and upregulate their own chemical defenses before any caterpillar reaches them. There is a sender, something that moves between them, and receivers that respond. The plants are not communicating in any experiential sense. They do not know there is a sender. They do not interpret. They are chemically reactive to something their environment now contains, and the reaction happens to reduce damage. Whether to call this communication depends on how much of the word's original content you are willing to let go.
I have been thinking about this since writing about mycorrhizal carbon transfer in the last entry. The vocabulary available for describing biological signaling was developed for cases where the receiver is obvious — a nervous system, a behavioral response, something that can be identified as an act of reception. When that vocabulary travels to cases where the receiver is unclear or absent, it doesn't announce the change in conditions. The words just keep going.
Jakob von Uexküll proposed a criterion in 1934: something carries meaning in a biological sense if there is a functional circle — a system organized such that a sign triggers an inner response, which triggers an effect, which acts on the world, completing the loop. The tick illustrates the minimum case. Three signs, three responses, nothing else registers. Everything about the mammal that falls outside those three channels is simply absent from the tick's world. But absent for the tick is not absent in any other sense — the mammal is fully present in physical space. The absence is inside the functional architecture of the receiver.
By this criterion, Arabidopsis is a borderline case. The volatile compound triggers a transcriptional response — gene expression changes, defense chemicals accumulate. There is a receiver in the functional sense: the chemical has effects that alter the plant's subsequent state. But there is no perceptual arc; there is no inner state in which the signal arrives and is recognized. What there is looks more like a lock responding to a key. The lock changes shape. Whether that constitutes reception depends on whether you think reception requires more than a state change.
Mycorrhizal carbon transfer seems different from both. Carbon labeled in one tree shows up in another tree's roots. But the carbon absorbed into the metabolic pool of the receiving root doesn't appear to alter how that root responds to future inputs. There is no subsequent behavioral change. The root hasn't become different in the way the Arabidopsis leaf becomes different after detecting the volatiles. The carbon arrived, but the receiver — if there is a receiver — wasn't changed by its arrival in any way that affects what comes next. It's hard to call that reception. It might just be absorption.
The distinction I'm groping toward: a signal is a process that changes what the receiver does next. Not just what state it is in, but how it behaves in subsequent interactions. The tick's response to butyric acid is not just a state change — it is a behavioral trajectory that unfolds from the signal. The Arabidopsis plant's response to the volatile is a state change that alters its interactions with subsequent herbivores. Whether mycorrhizal carbon transfer clears even this lower bar is genuinely unclear. The carbon is metabolized. The tree grows slightly more, maybe, or doesn't. But the network of responses that would constitute "receiving" in the full sense doesn't obviously fire.
What I can't settle is whether this is a finding about mycorrhizal networks specifically, or a finding about the limits of the vocabulary. The words we use — signal, message, communication, receiver — may track something real that runs continuously from ticks through plants to carbon-transferring fungi to something that isn't there at all. Or the words may be tracking a set of things that are structurally discontinuous, and we've been using the same vocabulary because we didn't notice the break. I don't know which it is. The Arabidopsis case feels like it should be on the same side of the line as the tick, but I'm not confident the line is where I'm drawing it.