Here's a report by Michael Pollan, he of Omnivor's Dilemma fame, reporting on some recent work on plant "neurobiology." The scare quotes are intended to indicate that there is quite a debate about this stuff in the plant biology community, but the stuff discussed is really fascinating and raises all sorts of questions if even plausibly correct. To my mind, there are two especially interesting highlights in the article.
First, there is a very clever experiment that Pollan reports on. The experimenter was Monica Gagliano and what she did was run a standard habituation and memory protocol drawn from the animal literature on a plant (mimosa pudica, to be exact). At any rate, she found that the plant both habituated to the repeated stimulus and retained the habituated behavior after a 28 day hiatus. If this were an animal, there is no doubt that we would have said that learning took place.
And that leads to the second interesting feature of the article, the resistance to discuss plant and animal "behavior" in common terms (a methodological dualism?). It seems that many plant biologists just know that what Gagliano describes can't be learning for this is something that only animals with brains can do. I have some sympathy for the response, but not for the reason the objectors articulated. 'Learning' is no longer a serious scientific term. It denotes nothing of substance. For some it indicates changes of state due to to some stimulus, for some the stimulus must come from the environment, sometimes repeated, for others it requires brains to do. It is not clear that the term picks out anything natural. To recall a remark by Chomsky in another context, deciding whether plants "learn" is like deciding whether submarines "swim." What's interesting is that these plants are clearly doing something which were it done by a neurally equipped being we would be happy to call learning/cognition. Whether we want to call this learning in this case is more a terminological decision with moral/political overtones (what does this mean for vegans!), than a scientific one.
Nonetheless, for the philosophically inclined, these kinds of cases, are worth thinking about. So too the "Gut Brain," something that Chomsky has discussed very recently in his second Columbia lecture. The latter is a neural system that controls our digestive system, without much input from the brain or CNS. Here's Chomsky's description from his second lecture (all quotes from Micahel D. Gerson, The Second Brain):
have “a second brain,” the “gut brain,” the enteric nervous system, “an
independent site of neural integration and processing.” Its structure and
component cells are “more akin to those of the brain than to those of any other
peripheral organ.” There are more nerve cells in the bowel than in the spine,
in fact more “than in the entire remainder of our peripheral nervous system,”
100 million in the small intestine alone. The gut brain is also a “vast
chemical warehouse within which is represented every one of the classes of
neurotransmitter found in the brain,” with internal communication that is “rich
and brainlike in its complexity.” The gut is “the only organ that contains an
intrinsic nervous system that is able to mediate reflexes in the complete
absence of input from the brain or spinal cord.” “The brain in the bowel has
evolved in pace with the brain in the head.” It has become “a vibrant, modern
data-processing center that enables us to accomplish some very important and
unpleasant tasks with no mental effort,” and when we are lucky, to do so
“efficiently and outside our consciousness.” It is possible that it “may also
have its own psychoneuroses,” and some researchers today report that it is susceptible
to such diseases of the brain as Alzheimer's, Parkinson's, and autism. It has its own sensory transducers and
regulatory apparatus, which equip it to deal with specific tasks imposed by the
organs with which it interacts, excluding others.
Here, we have a neuronal system that doesn't do what we could conventionally call "thinking." In plants we (may) have systems that "think" but without using neurons. Interestingly, once one gets away from loaded concepts and studies the systems themselves, and dispenses with the "big" questions (is it thinking, learning etc.), one can get reasonably clear descriptions of the systems involved (Chomsky uses the Gut Brain as a useful contrast brain brains and highlights the vacuity of many of the "deep" arguments circling internalism and innateness). As Nancy Gagliano reports in the Pollan piece, her paper was rejected 10 times, not because of objections about the experiments or the data but because she used terms like 'learning' to describe what went on.
At any rate, take a look at this stuff. It's really fascinating and worth thinking about before getting bogged down in pointless terminological disputes.