Sweet memories and sappy thoughts

I was just at the 2025 Botany conference in Palm Springs, California. A whirlwind event with talks on everything plants, from the history of botanical science to genomics of hybrid species. Returning from the conference on Thursday was quite a scene—a few hundred botanists packed into the Palm Springs Airport. I boarded the plane back to Knoxville (with a layover in Dallas); nearly every other passenger was a botanist.

An unexpected highlight of the trip was an engaging discussion I had on the plane. I was lucky enough to sit next to Dr. Jessica Savage, the leading scholar of phloem: the sugar conducting tissue of the plant. I initially planned to work on a paper I had been putting off for a while, but instead Jessica and I got to talking. The conversation kept going for the entire two and a half hour flight to Dallas, nearly exclusively about phloem.

Phloem is a fascinating tissue of the plant body. It is composed of large cells that use osmotic pressure to push sugary sap solution through the plant body. The cells that actually do the moving are called sieve elements, and through evolution they have rid themselves nearly entirely of their cellular contents. This means they lack a nucleus, mitochondria, Golgi apparatus, and nearly all the other organelles of a normal cell; it is much easier to move sap through a tube without a bunch of stuff in the way. Near these sieve elements are cells aptly named companion cells. These have all their normal cellular contents and help support the sieve tubes in their normal functioning. Each companion cell is like the life support machine of the skeletal sieve tube.

The way sugar solutions (sap) moves through the phloem is quite fascinating; please bear with me as I believe it is important to explain. Take a leaf of a Red Oak (Quercus rubra) tree during the growing season. The leaf is constantly pumping out sugar molecules through photosynthesis and those molecules are transported from the chloroplasts to the vascular system. They eventually enter the sieve elements. As sugar is constantly pumped into the phloem it creates a supersaturated sugar solution near the leaves. This means that water is going to want to rush into those cells through osmosis to balance solute concentrations. As water fills up those cells it creates positive pressure which pushes water towards areas of lower sucrose concentration (in this case it is down, away from the leaves). This simple process leads to the bulk flow of sap from the source (leaves) to the part of the plant with lower sugar solution (the sink).

Young leaves of Liquidambar styraciflua emerging in Spring.

While the osmotic mechanism of flow stays the same, the direction of flow across the plant can flip depending on the time of year. In many perennial plants in the temperate region during the early springtime, it is the roots that are the source of high sugar concentrations and the terminal twigs that are the sinks. Sap is thus pumped up in the reverse direction, from the roots to the shoots.

This process of sap flow and the reversibility of the flow is fundamentally different from the water conducting system of the plant (xylem) which—barring rare exceptions—always moves in the same direction, sucking water up from roots to leaves. Another fundamental difference is that the majority of the cells that move water in the xylem are dead at maturity, meaning they are completely hollow, relatively inert tubes. There is minimal regulation and little active response to the environment compared to the phloem.

The phloem is not just a mover of sugar molecules, but also a network for macromolecules like hormones, secondary defense chemicals, electrical signals, and more. The discussion with Jessica got me thinking about communication within the plant as an integrated individual.

Somewhat tired from staying up late the night before, I had a curious, maybe childlike, question: “is the phloem a sort of plant brain?” We laughed at the silliness of the question, but Jessica entertained me. We had an interesting discussion of plant sense, control, and integration across the individual, that was partially botanical and partially philosophical. She entertained my question and told me that the phloem is in fact how an individual sends signals from one part of the plant to the other in response to external stimuli and the phloem is even sensitive to touch; in some instances if you handle the plant a bit too rough sap flow could stop entirely.

The idea of a “brain” spread across an organism like tiny little filaments through the body, instead of a single centralized cluster of cells is maybe alien to our human-centric view. But, there are animals with these sorts of nerve strands throughout their bodies. For instance, jellyfish and other cnidarians have nerve nets spread across their bodies with no central brain. Starfish likewise have a similar nerve ring in their center with a radial system branching into each arm, in some capacity these arms can operate individually. These diffuse nervous systems allow these animals to complete quite complex tasks without any centralized system, and while very different in structure, chemistry, and physiology, somewhat echoes how an individual plant may send signals within its own body.

To be clear, I am not saying that plants have brains. Plants cannot “think” or “feel” or “emote” the way animals do. Indeed, there is a whole subset of research called "Plant Neurobiology”, which seems borderline pseudoscience. The papers sometimes read feverish and discuss similar ideas to the one I espouse here but with stronger claims of neuronal activity in plants. And there are many valid critiques to this way of thinking. The point I wish to make is simply an analogy. The phloem is a highway of connectivity across the individual where the organism senses its surroundings, sends signals to another organ, and responds. It is not a brain, neurons, or a neural network in the way we think of it, but it is a fascinating system of interconnectivity across the organism.

I think it is natural for us to try to anthropomorphize plants and other organisms. We try to connect with them in the ways we know best (e.g., how are we similar?). However, plants are not like us, they are fundamentally different from us. They do not have a centralized nervous system, they don’t eat like us, breathe like us, develop like us, live like us, or die like us. But, we can appreciate them all the same. Not because we can find ways that they are similar, but because we know that they are so fundamentally different. We can find connection in the fact that we share a common ancestor with these wildly different organisms, some 2-3 billion years ago.

August 10, 2025

References

Esau, Katherine. "Development and structure of the phloem tissue." The Botanical Review 5.7 (1939): 373.

Heydari, Sina, et al. "Sea star inspired crawling and bouncing." Journal of The Royal Society Interface 17.162 (2020): 20190700.

Knoblauch, Michael, et al. "Testing the Münch hypothesis of long distance phloem transport in plants." elife 5 (2016): e15341.

Robinson, David G., and Andreas Draguhn. "Plants have neither synapses nor a nervous system." Journal of Plant Physiology 263 (2021): 153467.

Savage, Jessica A. "It's all about timing—or is it? Exploring the potential connection between phloem physiology and whole plant phenology." American Journal of Botany 107.6 (2020): 848-851.

Watanabe, Hiroshi, Toshitaka Fujisawa, and Thomas W. Holstein. "Cnidarians and the evolutionary origin of the nervous system." Development, growth & differentiation 51.3 (2009): 167-183.