The Quiet Branches

Recently, Evolutionary biology PhD candidate and science communicator Sally LePage discovered that 10 of 12 water companies in the UK use divining rods. It was picked up by The Guardian amongst other places.

Phantom Sense

Divining rods, or dowsing objects, do not work. They are no better than chance and Le Page cites the evidence and discusses what underlies the illusion: the ideomotor effect combined with commonality of water under the surface of the Earth and confirmation bias in the human brain.

Plants have underground roots, they live in the ground, and that is how water gets taken in and people would have realized that quickly.

It is a bit shocking that dowsing is still a thing used by any professional in 2017. It’s not just water companies in the UK. It occurs all over.

Modern dowsing goes back about 450 years and it’s not just water that people looked for in the Earth. Minerals, gold, oil, and anything valuable has invited dowsers. It’s part of a suddenly striking it rich fantasy that many humans seem to carry deeply in our minds. The American Society of Dowsers suggests that the practice is more ancient, going back thousands of years (though a definitive history of it does not seem to exist).

Early dowsing rods were made tree branches. However, metal rods, pendants, and more have been used as well.

Using part of a plant to divine water resembles an ancient medical idea of “like healing like” (the idea that if something resembled an organ or disease in some way, it would treat it – also spurious). People likely noticed early on that places with water were lush with plant life, and noted some relationship must exist there. Indeed, as living things, water is essential. That part of life is hard-baked in to the DNA of every living thing on Earth – don’t dry out, or if you do, have a strategy to cope with it – like the desiccation-tolerant tardigrade or resurrection plant do (both are champions of surviving with little water).

Plants have underground roots, they live in the ground, and that is how water gets taken in and people would have realized that quickly. It’s not that far a leap to make that part of a plant might lead one to underground water. Especially when combined with other human mental quirks, it’s easy to see how a narrative could take hold. It’s at least a place to start exploring how best to find water.

To be clear, this may well be a “just-so” story. I can’t find evidence that this is how dowsing evolved. However, it is possible.

Biological Water Sensors

Living plants really are able to detect water and respond to it. Either foraging for it in the soil or responding to flood waters. Plants need some water and have biological sensors to detect it and know if the entire plant needs more or less water at a given moment.

It may seem obvious that roots are a complex network – there are lots of “root brains”.

Scientists study just what mechanisms underlie water sensing, or hydrotropism. It’s akin to a plant growing towards sunlight, phototropism. Tropisms require detection of an environmental gradient. A way for cells in plants to signal they’ve spotted something different in one area vs. another, and then to grow preferentially toward or away from where there is more or less of the gradient substance (e.g. water or light).

Another powerful tropism in plants is gravity. Roots are positively gravitropic for the most part (growing towards gravity) while shoots are negatively gravitropic (growing away from gravity). There are exceptions, like peanuts that bury their fruit in the ground after developing from flowers above it (geocarpy).  Most roots have a sense of gravity and can use it to orient themselves and as a initial cue when a seed germinates to send a root down which is generally where water can be found. Gravity tends to be a stronger tropism than hydrotropism. Tropisms have to be weighted and balanced to environmental and developmental conditions. When scientists say plants are sensitive to their environment, this is what they mean. The calculations that have to happen inside plant cells to make sense of its context are complex.

The cells responsible for hydrotropism in Arabidopsis (the “lab-rat” of plant biology)are near the growth front of roots. Just behind the stem cells, is a special place in roots where cells go from dividing to elongating and specializing into their final roles (see figure above). This is the region Darwin called the “root brain”. In this region, direction of root growth is determined by shuffling hormones across root cell files causing differences in growth from one side of the root to the other, thus turning the root towards or away from a sensed gradient- be it gravity, water, or nutrient – all of these have to be balanced. For instance, gravity is sensed in the apex of the root, in the root cap and the hormone auxin gets redistributed and a root can then bend.

How One Plant Senses Water

In Arabidopsis, hydrotropism is independent of gravity and the water sensing takes place in a specific file of cells known as the cortex (see figure above). It’s the row of cells just inside the epidermis (akin to our skin…it would be the cell layer just below the one exposed to the environment). It also relies on an important plant hormone…not auxin in this case, but abscisic acid (ABA). ABA is the plant hormone most connected to drought and heat and water stress in plants. ABA “talks” to auxin (as all hormones do – part of balancing tropisms), however, based on recent work, it seems sufficient on its own to get a root to bend toward more water.

Dietrich et al. figured out the cortex cells behind the meristem were responsible for hydrotopism by first destroying the gravity sensing cells in the root apex (columella in figure above) and then the entire meristem with a laser. In both cases, the part of the root remaining was able to bend along an artificially set up water gradient. They then did some clever molecular biology, expressing a gene involved in ABA signaling and one specific to hydrotopism called MIZ1 in specific types of root cells, including the cortex. Whenever this gene was expressed in the cortex, hydrotropism occurred. When it wasn’t, it didn’t. The researchers did a number of other experiments to support their claims, including disabling the hydrotropic cells by making them elongate more than normal. This speed up of normal developmental time prevented hydrotropism from happening.

Gagliano et al. published a paper earlier this year indicating can also “hear” water. This may explain the phenomenon of people finding roots growing around sealed water pipes, or at least bending towards them. Even a recording of water could get plants to grow towards it, which was not the case for white noise.

A Complex Relationship

Most plants don’t blindly grow towards water, but optimize their water needs with the rest of the plant’s needs and generally seek to avoid being flooded. Plants have to prioritize their tropisms, being able to tune their responses as well as possible.

Once the evidence is in, changing one’s mind and learning how plants actually sense water at the cellular level is the way of science, of progress in understanding the natural world.

It may seem obvious that roots are a complex network – there are lots of “root brains”. One might find a rich source of phosphorous, sending it to the rest of the plant, letting the entire plant know “we’re good on this resource, find water, because we really need that” to other parts of the root system. Resources are often asymmetrically distributed in the soil. Another complexity: soil fungi, microbes and other plants form a network where at least some resources can be shared or passed around in what amounts to a trade network. These networks form the primary production ecosystems – and we humans – rely on to live.

There are 400,000 species of plants on Earth and they are diverse, with their own specific biology, and conditions (some plants don’t have roots anymore). Their hyrdotropic responses likely vary in mechanism and degree. Some plants, like Zostera, that live in the ocean may not be hydrotropic at all. And plants like coast redwoods rely on water can come from non-underground sources like fog, like coast redwoods.

Dowsing is a pseudoscience. It is, however, perhaps understandable how people came up with a narrative that using part of a plant that clearly need water to live and get it from underground might lead people to water. Once the evidence is in, changing one’s mind and learning how plants actually sense water at the cellular level is the way of science, of progress in understanding the natural world.

References

Peret, Benjamin (2017): Primary and lateral root.ai. figshare. https://doi.org/10.6084/m9.figshare.5143987.v4 Retrieved: 17:49, Dec 03, 2017 (GMT).

Dietrich D., Pang L., Kobyashi A., et al. Root hydrotropism is controlled via a cortex-specific growth mechanism. Nature Plants 3, Article number: 17057 (2017) doi:10.1038/nplants.2017.57

Gagliano, M., Grimonprez, M., Depczynski, M. et al. Oecologia (2017) 184: 151. https://doi.org/10.1007/s00442-017-3862-z