RHS Journals
The Garden
November 2007
Out of sight, out of mind
Although not often seen, roots are just as vital to plants as leaves and flowers. Andi Clevely uncovers how they have evolved into a variety of forms for different specialist functions
Most roots live underground and out of sight, so their importance is often overlooked, at least until suckers, subsidence, perennial weed invasion or a debilitating root ailment reminds us of their existence. Even then, few gardeners are aware of the crucial roles they play in the life of plants.
Roots are one of evolution’s great leaps forward. The prehistoric world first turned green when a congenial climate allowed marine plants to creep ashore and exchange a drifting existence at sea for a settled life rooted to the spot. There they were exposed to more sunlight, which in turn supported increased size and diversification into the numerous plant genera around us today.
That was in the Silurian period, between 408 and 438 million years ago. Since then plants have colonised almost every part of the globe, developing an arsenal of specialised root types as they adjusted to various habitats.
Principal functions
Anchorage remains one of the chief activities of roots. Right from the moment when a dormant seed awakes into life, its mass of excited cells bursts the seed coat open to liberate – tellingly, first – an infant root that extends downwards and braces the seed ready for its shoot to emerge.
This primary root or ‘radicle’ can grow up to 2cm (3/4 in) per day, its tip protected against wear by an apex of hardened cells. Behind this streamlined root cap is a region of rapidly dividing and extending cells similar to those at the tip of an active shoot. Some cells are sensitive to gravity and steer the root downwards; others behind the tip develop a zone of fine hairs with a huge surface area that make intimate contact with soil particles, dramatically increasing the root’s ability to absorb nutrients.
Locating and taking up food and water to power plant processes is the other main role of roots. Any plant is essentially a ‘fuel refinery’ with an efficient distribution system. With the aid of chlorophyll – the green pigment without which none of us would be here – leaves transform sunlight into chemical energy in the form of sugars. These are transported down the stems to power roots in their search for water and dissolved nutrients, which are pumped back upwards to support the metabolism of the above-ground parts of the plant.
To function fully, however, a plant needs more than a single primary root. Sometimes the radicle delves ever deeper as a fattening taproot, branching as it lengthens to explore the adjacent soil. This is the typical root system of dicotyledons (‘dicots’), plants such as pelargoniums and dandelions that have two seed leaves or cotyledons. Those with one seed leaf, monocotyledons (‘monocots’) – such as grasses and orchids among others (and a few dicots such as strawberries) – replace their short-lived radicles with a system of branching, fibrous roots of similar size growing from the stem base.
Modified roots
Root types are often influenced by the specific habitat or growth pattern. Trees develop powerful taproots to extract huge daily volumes of water; scree plants often have deep taproots that resist earth movement; and many cacti develop a wide-ranging fibrous network near the surface to quickly intercept dew and rare rain. Plants conquer challenging situations with more specialised roots.
The type of roots a plant has plays a vital part in determining its success or failure in a particular habitat. Even tiny, floating Azolla ferns depend on dangling roots to filter dissolved minerals from pond-water, and carnivorous plants, with their ingenious alternative food supply in nutrient-poor rooting conditions, still need anchorage.
Most roots are never seen. Digging up a plant leaves the majority of its root system behind and usually reveals little of its functions or extent, which can be enormous: in one famous experiment, a four-month-old rye plant produced a total root length of 622km (386 miles), with root hairs estimated at more than 10,000km (over 6,200 miles). Species of Liatris can penetrate 5m (17ft) deep in their desert home, tamarisks may go down 50m (170ft) in the desert, while horsetails and field convolvulus can be found in ordinary gardens more than 2m (7ft) deep.
Just being aware of their diversity and the many dynamic roles roots play, however, can help us appreciate that the simplest plant is a triumph of organisation and deserves all our efforts to help it put down roots contentedly in the garden.
Read more about different types of roots
The Root Zone at Wisley
Beneath the waterfall in The Glasshouse, RHS Garden Wisley’s new attraction, formally opened by HM The Queen in June this year (see The Garden, June, pp394–399 ), is an area revealing much more about roots than you can discover with a spade.
The Root Zone demonstrates the many ways in which plant roots interact with the soil. The room is dominated by a model of huge tree roots hanging down from the ceiling, while the walls are lined with touch-screen displays and samples of real roots.
Monitors show time-lapse images of root systems, and portholes offer glimpses of products developed from roots – foods, medicines, dyes, spices and flavourings, among others.
Intended as an educational facility, the exhibition also features poems and quotations; one of these – ‘We know more about the movement of celestial bodies than about the soil underfoot’ – quintessentially sums up the whole purpose of The Root Zone.
Find out more about The Root Zone in The Glasshouse