The following is an extract from our nature newsletter Wild Wild Life.ĚýSign up to receive it for freeĚýin your inbox every month.
I’ve been thinking a lot about roots lately. In the seven years I spent studying and researching plants at university, there was always something of a bias towards the easier-to-see parts of a plant – the stems, leaves, flowers and fruit. My doctoral work focussed on the genetics of fruit and seed dispersal, but during one of my experiments I remember a moment marvelling down a microscope at an embryonic root sitting patiently in wait inside a tiny seed. This – the brilliantly named “radicle” – is the first part of a plant to push itself out of the seed upon germination, venturing out into the soil and preparing a newly formed plant for all that is to come.
We all know that plants have roots and that these are important, yet their lives in the dark beneath us make them somewhat mysterious. So, in the interest of sharing some of their secrets, here are seven things you may not know about roots.
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1. Not all plants have roots
For most plants, roots are fully essential. They provide plants with water and nutrients, as well as holding them in place. They can store up resources for surviving through winter or drought, and they can also help some plants spread and reproduce, sending up new clones nearby. But some plants have evolved to survive without them.
A recognisable example is Spanish moss (Tillandsia usneoides), the grey-green tangled chains of leaves and stems that grow hanging from some trees in parts of the Americas. The plant takes its moisture from the air, and it is not technically a moss, despite its name – it is a flowering plant in the same family as the pineapple, and its relatives have roots, although often not many.
Spanish moss’s way of life has enabled it to lose its roots over time, but real moss never evolved roots in the first place. Mosses are in a group known as the bryophytes and these all lack roots, absorbing water through their leaves instead.
It clearly works for them, but the fact that these rootless plants usually need to grow in damp environments hints at how beneficial roots are in all but the moistest environments.
2. Roots sense their world
Like other plant parts, roots are sophisticated at sensing the world around them, growing towards both moisture and gravity, and away from light. But surprisingly, a lot of the details around how they do this remain unclear.
Gravity-sensing is particularly intriguing. It’s thought that roots do this using statocytes, specialised cells that contain statoliths, themselves a specialised type of starch-storage structure. This may sound unpromising, but the idea is quite elegant – heavy starch granules inside these statoliths make them sink under gravity and, somehow, statocytes are able to detect in which direction their internal statoliths are moving. How they do this, and how this then prompts roots to grow towards gravity, seems to involve the hormone auxin, but we don’t fully understand it yet – although experiments in the microgravity of the International Space Station are helping.
3. Roots need help from their friends
My space-expert colleague Leah Crane recently wrote a fascinating newsletter about all the surprising reasons it’s difficult to grow plants in space. One issue is the substrate – moon dust and Mars dirt are toxic and lack nutrients. But a team has managed to grow chickpeas in lunar soil by adding worms to provide nutrients and fungi to remove toxins. This achievement highlights the importance of what’s known as the rhizosphere, the zone of soil that surrounds and is directly influenced by plant roots.
The rhizosphere is home to many microorganisms, good and bad. It can be home to disease-causing types of fungi, bacteria and tiny nematode worms, as well as bacteria that help stimulate growth, provide nutrients and kill pathogens. Symbiotic fungi known as mycorrhizal fungi help extend the reach of plant roots further into the soil and improve their uptake of nutrients, particularly phosphorous. This hidden microbial ecosystem has been neglected by many gardeners and farmers – conventional methods for working the land disrupt and inhibit the root microbiome, leaving plants dependent on us to feed them and fight their diseases for them.
4. Roots are strong
It may seem unsurprising that roots are strong – they anchor huge trees in place, after all – but their strength is on display even among more humble plants. The leek, for example, is pulled down into the ground by its roots, which shrink and contract, bringing the leek downwards until the roots detect adequately low levels of blue light. Horseradish, meanwhile, has a notoriously vigorous root that grows fast and deep. Once horseradish roots are established in an area, it can become invasive and very difficult to eradicate, continually regenerating from fragments of root left deep behind. For this reason, gardeners are advised to grow horseradish separately, in a pot, but even then, its roots have apparently been known to exit through the bottom and force their way through the paving underneath.
5. Roots need oxygen
Despite being connected to stems and leaves above ground, roots need their own local supply of oxygen. While water and sugar are transported throughout a plant, oxygen is not, and without this, roots cannot respire or function.
It would be easy to assume that the role of roots is quite passive – water and nutrients flow into them and up through the plant. But this isn’t the case for vital mineral ions, which root hair cells must actively pump into the plant from the soil. This process uses specialised proteins and energy, and to release that energy, root hair cells need oxygen.
This is one reason why water-logged soil can be fatal for plants that aren’t adapted for life under water. When plant roots are submerged for too long, they run out of oxygen and suffocate.
6. Roots can “walk”
We have many idioms based around roots, from discussing where we came from (“going back to my roots”) to talking about making a home (“putting down roots”). Often, these metaphors riff off the function of roots to hold a plant in place (“she stood rooted to the spot”), but there’s a renowned example where roots do the opposite, enabling a plant to move as if walking.
The walking palm, Socratea exorrhiza, (pictured above) is famed for growing new roots out of its trunk. Over time, these roots come from higher and higher up the trunk, and the older roots may die off or rot away. If the trunk bends for some reason – towards light, or after being flattened by falling debris – the progressive death and growth of roots from it can result in the whole palm moving away from the site where it initially germinated and straightening up to stand some distance away. It’s thought that these palms may be able to “walk” approximately 1 metre per year.
As a former plant developmental biologist, I have to note that the walking palm’s “roots” are technically not true roots, because they grow from the stem of the palm, not from its original root tissue. Roots like these are called “adventitious” roots and are common, grown by plants as diverse as maize, rice, bamboo and orchids.
7. Some plants exist mostly as roots alone
Returning to plants that grow on other plants, or epiphytes, while some like Spanish moss lack roots, others, like the ghost orchid (Dendrophylax lindenii), are pretty much only roots, most of the time. Growing in Cuba and southern Florida, this orchid lacks leaves, and its roots are green, having taken on the task of photosynthesis that is usually performed by leaves. This rare orchid lives as roots that wrap around the trunks of their host trees, but they do sometimes flower, producing sweet-smelling white blooms in the shadows of the forest.
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