© NHPA

 

 

‘Unlike the soils of many moist habitats, where water and nutrients are in plentiful supply, the soil of deserts lacks organic matter.’

 

Life in the Extreme

The world’s arid environments, which comprise the planet’s desert biome, are so extreme that the plants that survive in them are some of the most highly evolved on the planet. Though plants have their origins in the ancient lakes and oceans of the Earth, innumerable adaptive steps have enabled some to become modified to a life further away from their aquatic beginnings, over time colonising drier and more inhospitable lands.

Since the rise of plants in Earth’s oceans over a billion years ago the climate has fluctuated through prolonged cold glacial periods interspersed with warmer interglacial periods. The most recent period of mild temperature has roughly spanned the last 10,000 years of human civilisation. But contrary to this short- term trend of warming, on a broader timescale of the last 60 million years the planet has seen a pattern of increased levels of glaciations. As a result, huge areas which were once green and tropical have become more barren, arid landscapes. In response to the increasingly arid climate, plants from previously humid habitats evolved to cope with extreme climates. For example, a particular jungle thorn bush called Pereskia developed many characteristics by which modern cacti are defined today – long spikes emanating from its tall woody stem, thickened waxy leaves to store water, and the ability to respire using a chemical pathway called crassulacean acid metabolism (CAM photosynthesis), which allows the plant to open its stomata to absorb carbon dioxide only at night, therefore minimising water loss. This plant first evolved roughly 50 million years ago, and it is believed to be the ancestor of all cacti, perhaps the most extreme group of dry-zone plants adapted to life in the desert.

Over time many new drought-tolerant plants radiated, each possessing different adaptations to life with little water. Plants from the family Aizoaceae or ice plants evolved to live in the deserts of southern Africa; they adapted to grow largely underground to stay cool and minimise water loss, and they developed their strategies of pollination and reproduction to suit the seasonally scorched landscape. A family of plants called Didiereaceae which are endemic to Madagascar developed thickened water-storing structures in their leaves and tall thin stems. Another family of plants from the arid regions of the southern hemisphere called Portulacaceae developed thick leaves and stems to retain water, while the Cactaceae family from the Americas evolved into some of the most iconic desert plants known today. These dry-zone plant families diversified over the past 40 million years, and as modern continental positions and climates have become more stabilised over the last five million years, they evolved into the plant species which fill the Earth’s arid habitats today.

Increasing ice deposits in Antarctica as well as the formation of mountain ranges caused further aridification of certain regions of the planet. The colossal uplift of the Andes formed a rain-shadow along the spine of South America and ice build-up at the South Pole caused the creation of a cold offshore Peruvian current. Today the Atacama and Sechura deserts of Peru form the driest landscape on Earth, including one of the few places where rain has never been recorded. The same effect was seen in the northern hemisphere, where the formation of the Rocky Mountains blocked coastal moisture from reaching the mid-continent, starving the modern Great Plains and the Mexican Plateau of moisture. Similarly the uplift of the Tibetan Plateau led to the desertification of large parts of Asia and is believed to have contributed to the increasing aridity of the Sahara.

Babies’ toes
These plants from the Aizoaceae or ice plant family store water in their thickened leaves.
© RBG Kew

Today deserts make up about a third of the surface of the planet, comprising an area of around 80 million square kilometres. They are characterised by their extreme temperatures and near-absence of precipitation, which is usually less than 20 centimetres a year, compared to the 200 centimetres that fall on the Amazon basin. The dry winds which shape the surface of the landscape like sandpaper increase the aridity further still. Unlike the soils of many moist habitats, where water and nutrients are in plentiful supply, the soil of deserts lacks organic matter. In some cases the environment for survival may be so limited that many of the plants that live there stand alone in order to reduce competition between them. However, although these desert areas appear to be barren wastelands compared to tropical forests, they are in fact biologically rich habitats.

Each desert environment around the world is defined by its own plant life. The Namib Desert of southern Africa is believed to be the world’s oldest continuous desert, dating back to around 55 million years ago. Today it contains a number of plant species which have evolved unique traits in order to survive in this relentless climate. Welwitschia mirabilis can live for many thousands of years, existing as no more than a pile of dried, brown leaves. Pachypodium namaquanum – or elephant’s trunk – grows as a 4-metre-tall, bare totem-pole topped with a few fleshy leaves, and quiver trees (Aloe dichotoma) rise out of the gravel scrub resembling a giant bouquet of miniature palm trees supported by a main trunk wrapped in sharp bark. In America’s Sonoran Desert saguaro cacti dominate the landscape with their 21-metre-tall arms enshrouded in a cloak of spines, viciously spined agaves store water in their succulent leaves, and a sea of vivid brittle-bush flowers flood the desert floor from March to June. The expansive sunburst red sands of Western Australia cover just under half of the continent and are punctuated with short scrubby tufts of spinifex grass, saltbush shrubs, and yellow flowers of the nation’s emblematic wattle trees.

Quiver tree
This desert dweller gets its name from the local tribes, who use its bloated branches to make containers for their arrows.
© RBG Kew

Whether it is hot and dry or cold and dry, a plant’s primary problem in these dry zones is coping with lack of water. For large parts of the year these environments are either baked by the sun’s rays, or they are whipped by sub-zero winds which lock up any moisture in ice. The waxy cuticles of most vascular plants act as a barrier to prevent water loss in temperate climates. When these plants lose a small amount of water from their leaves via transpiration it is soon replaced by the process of capillary action which eventually wicks more water up from the plant’s roots. However, in hot arid habitats where water is not readily available in the substrate surrounding a plant’s roots, there is nothing to replace lost water. Dry-zone plants have evolved numerous mechanisms and structural modifications to deal with the scarcity of water, both in minimising water loss and in developing bodies which can store water. Collectively these drought-tolerant plants are called xerophytes, more often collectively known as cacti and succulents.

Some desert areas may never see rain at all, but most deserts do at least receive a few centimetres of precipitation a year. This typically falls in sudden torrential downpours, so the key to plant survival is to store up as much water as possible. The first step taken by many xerophytes to achieve this was to evolve succulent water-storing body parts in the form of thickened structures composed of multiple layers of cells. The leaves of Echeveria laui from Oaxaca in Mexico have evolved to become water-storing parts, resembling swollen pink and white paddles. The cacti from the Americas have reduced their leaves to become nothing more than spines, to prevent water loss, and their stems have evolved to be able to photosynthesise as well as store water. The golden barrel cactus (Echinocactus grusonii) from Mexico’s Moctezuma River canyon, whose spherical form is the ideal water-storing shape, minimises water loss through transpiration. Many cacti, together with the euphorbias, their Old World equivalents from Africa, are also covered in a thick flexible skin with pleats which run down the length of their bodies, allowing them to expand and shrink like a concertina as they absorb and use up water throughout the year. This thick skin also insulates the plant from searing ground temperatures, which can reach as high as 60°C, and prevents their stored water from evaporating into the atmosphere.

The caudiciforms have specialised roots or bases in which they can hold large quantities of water. These can range from small but beautiful spindly growths of the elephant’s foot (Dioscorea elephantipes), whose deeply fissured root mass could be mistaken for a pile of chopped wood, to baobabs (Adansonia digitata), which grow in the savannahs of sub-Saharan Africa. The baobabs are often referred to as upside-down trees, as their relatively short and fat upper parts look as if they should be the tree’s roots, not its branches.

While cacti and other succulents are adept at absorbing and storing large amounts of water, they still need to open up the stomata in their thick skin to absorb the carbon dioxide necessary for photosynthesis, and this makes them vulnerable to water loss. To avoid opening up their stomata in the heat of the day, many dry-zone plants have evolved a method of opening them only at night, when evaporation rates are low. However, as they are unable to photosynthesise in the dark they have to store up the absorbed carbon dioxide until the morning. They do this by converting it into a chemical called malic acid. The following day this acid is broken down to release the carbon dioxide, and in this way the plant is able to photosynthesise with its stomata closed. Through this mechanism, known as crassulacean acid metabolism (CAM), desert plants lose 90 per cent less water per unit of carbohydrate synthesised than a non-CAM plant. As well as being super-efficient at retaining moisture, CAM plants also have the added advantage of being able to shut down their metabolism during times of extreme drought. When water is in short supply the plant’s stomata can remain closed permanently both day and night. The moist environment inside its cells allows a very low level of metabolism to keep going using nocturnally fixed carbon dioxide. The plants can remain in this state until water returns to their habitat, and they are able to regain their normal metabolic rate in just 24 hours.

Ultimate storage
Spherical cacti are the ideal shape for storing water, as they have the lowest possible surface area relative to their mass.
© RBG Kew

For the short time that water is available it is necessary for desert plants to have a highly adapted root system to absorb as much water as possible before it evaporates. Where plants in the tropics and temperate regions have long roots designed to reach deep into the ground to absorb stored water, plants in arid habitats have relatively shallow roots which sit just under the surface to suck up rain as soon as it falls. While cacti roots are not deep they are still extensive, and a metre-tall plant can have roots which extend out 3.5 metres horizontally – on average the roots of most dry-zone plants cover an area up to twice that of their canopy above ground. Cactus roots are covered in a springy cork-like coating which prevents them from losing water, and over the rainy season they put out many new root hairs to increase water uptake. After rainfall, when the ground begins to dry and crack again, sections of the roots die off – a small sacrifice to prevent the plant as a whole losing too much water. Many plants also drop their leaves during periods of extreme drought. To prevent water being lost via transpiration through their leaves, water is first drawn back into the plant’s stem and the leaves simply dry up and fall off. The creosote bush which grows in sporadic tufts in America’s Sonoran, Chihuahuan and Mojave deserts is unable to drop its leaves but instead covers the surface of its leaves with a thick resin which seals in the moisture.

However, some of the most extreme dry-zone plants, which often have to endure up to a year without water, don’t attempt to resist water loss but instead have evolved the capability to almost completely desiccate. One such plant is the small Selaginella lepidophylla, or resurrection plant, that grows natively in Mexico’s Chihuahuan Desert. This primitive plant is a lycopod, a small moisture-loving plant. During a period of drought it becomes a shrivelled brown ball of dried leaves no larger than an orange, and its metabolism slows almost to a stop. In this shrunken form its surface area is drastically reduced so that some moisture may be retained, locked in the centre of the ball. The plant lies dormant, often staying in this state of suspended animation for many years. Eventually the ground surrounding the resurrection plant will begin to saturate with rejuvenating drops of desert rain. As water is absorbed by its dried body, the plant’s metabolism restarts. Within a few hours the brown leaves begin to unfurl, plumping up as they fill with water, and fan out into a broad rosette of dark green branches 30 centimetres across. Its giant lycopod relatives were the dominant land plants 300 million years ago, but they have since gone extinct, and only the remarkable resurrection plant lives on.

Resurrection plant
Not only can this plant almost completely dry out, but it can remain that way for years, before miraculously coming back to life.
© Rob Hollingworth

Resurrection plant
As water returns to its habitat it can resume growth within 24 hours, stretching out flat, scaly, 30-centimetre-wide stems.
© Rob Hollingworth

The lycopods to which Selaginella lepidophylla belongs are the oldest living vascular plants, not dissimilar to the first plants that grew in the coastal habitats of the Silurian. Selaginella does not produce complex flowers, or seeds or pollen – instead, it produces spores – and the mesophyll layer of its leaves is only a few cells thick.

An angiosperm which is equally able to ‘resurrect’ is Blossfeldia liliputiana – a nod to the race of people encountered by Gulliver in Swift’s classic tale and the smallest species of any cactus. This tiny plant, whose grey-green stems reach no more than 2 centimetres in diameter, is fairly widespread in the arid habitats of the east side of the Andes, including southern Bolivia and southwest Argentina. It is wedged into cracks between rocks and when it dries up its swollen body shrinks and becomes flattened like a disk. Unlike all other cacti Blossfeldia almost completely lacks stomata, but instead has breathing pores called areolar pits sunk deep into little depressions across its surface. It is from these structures that the spines of cacti grow. It is believed that Blossfeldia has the fewest stomata of all photosynthesising plants on the planet, a trait which has clearly evolved in response to the extreme life strategy of this plant. Another trait unique to Blossfeldia is that it has no thickened outer cell wall and so it does not retain water like other cacti, but instead it allows its body to completely desiccate. In its dehydrated state this plant can wait for many months. But when rain does eventually return it can refill its cells and plump up its body once more, and within a matter of weeks it can produce beautifully delicate white flowers.

Mighty saguaro
These cacti are some of the most easily recognised desert flora.
© SuperStock

For the rest of the plant world, whose cells would simply shrivel and irreversibly deform if they allowed their bodies to dry out, the key to staying alive in extreme heat and drought is to become a master at retaining precious water stored in specialised body parts. Some of the most effective examples of such adaptations are best exemplified in the cactus family which – aside from one species, the mistletoe cactus from Asia and Africa – all originate from the deserts of the Americas. Acting similarly to the expandable ribs seen on the golden barrel cactus, many cacti such as Thelocactus from the scrublands of Mexico have structures called tubercles which are cone-shaped protrusions covering the plant’s surface, allowing it to expand and shrink without bursting its skin. Another Mexican species, Stenocactus crispatus, is covered in a deeply ridged sinuous ribbing which gives it the appearance of a strange green coral. Both the tubercles and the wave-like ridges increase the cacti’s surface area for absorbing sunlight without overly increasing the risk of water loss. In species where the ribs are most pronounced – creating 5-centimetre-deep pleats down the entire cactus’s body – this also provides the plant with a way to reduce the heat of direct sun, as when the sun shines from any side the plant’s ridges will mutually shade the rest of the plant. Another way for a cactus to reduce the glare of the sun is to reflect it away from its stem, and some cacti achieve this by covering themselves in a powdery layer of sunscreen, such as Pilosocereus pachycladus, whose body is covered in a stunning azure blue coating.

One way for a plant to reduce the amount of sun that it absorbs is to escape it completely, opting for a life underground, or at least partially so. This semi-geophytic existence helps keep the plant cool and in doing so reduces the amount of surface exposed to the drying rays of the sun. Some examples include the slow-growing spineless cacti from the genus Ariocarpus that have evolved to live with only the tips of their fleshy star-shaped tubercles above the ground, while the rest of their body sits buried in the soil. Another succulent plant called Fenestraria – or window plant – from the ice plant family has evolved bulbous green tube-shaped growths which sit in the substrate with only their tops showing. At the top of each tube-like growth is a flat area of translucent tissue which works much like a camera lens, moderating the amount of sunlight which the plant absorbs for photosynthesis. In the deserts of Brazil and Chile the sun is so powerful that it can even penetrate the translucent quartz rocks which cover the ground; using this cover to their advantage, the endangered Discocactus horstii can be completely covered by these rocks and still absorb enough solar energy to effectively photosynthesise, often only revealing itself above ground when it puts out its bristly white flowers to attract hawk moths.

The impregnable covering of sharp spines that have evolved from reduced leaves which enshroud most cacti are often wrongly assumed to be purely for protection, but in fact they also play an important role in controlling the temperature and moisture content of the plant. In species such as Pachycereus schottii, often known as the whisker cactus, the spikes grow tightly packed together, forming an insulating mat which helps shade the stem and traps humidity close to the plant to reduce water loss by transpiration. In many species of Mammillaria cacti this dense spination has evolved even further to create tufts of woolly hairs which surround the plant’s areoles creating a moist environment close to the plant’s stem. These spiny hairs also help protect the delicate flowering parts from drying out and from hungry predators when they are in bloom. In the most extreme examples dense mats of spines grow so thickly over the stem of a cactus that it creates its own microclimate underneath. Even in the heat of the day this keeps a layer of moist and relatively cool air around the plant, creating an efficient boundary against water loss. In some deserts, where the only source of water is in the form of a coastal fog which drifts on the breeze, cactus spines can act as a fog trap. Under the magnification of a scanning electron microscope one can see that the surface of the needle-like spikes of the Eriosyce paucicostata cactus from Chile are not smooth but actually covered in fine ridges and rough channels. Their coarse surface collects fine droplets of water from the air, and as they build up they drop on the ground below to be absorbed by the plant’s shallow roots. Some cacti, such as the Eulychnia species, even have lichens growing on their long thin spikes which help them to collect moisture from fog. Along the coastal parts of the Atacama Desert it is not uncommon to find these tall columnar cacti almost completely enveloped in a thick cloak of lichens. Some cacti growing in the Atacama have rough waxy structures on their surface which trap droplets of fog and channel them down towards the base of the plant; plants like the star-shaped Astrophytum myriostigma and the sea-urchin-like Copiapoa cinerea cover their entire bodies with this texture, invisibly collecting water in a land which appears to have none.

Window plant
The tips of this plant’s succulent leaves are made of translucent tissue acting a bit like a lens to regulate the amount of light it absorbs.
© RBG Kew

The water-capturing and water-storing capabilities of xerophytic plants – in an environment largely devoid of water – makes them an obvious target for any thirsty creature which resides in the dry zone. Desert plants provide a whole array of herbivores with food and water. Consequently over millions of years of evolution these plants have developed a number of defence mechanisms. The most subtle of the dry-zone plants are small enough that with suitable camouflage they can grow unnoticed by passing herbivores. This is the strategy adopted by a group of small ice plants called Lithops, which grow widely from sea level up into the mountains in South Africa and Namibia. With a name derived from the Greek lithos, meaning ‘stone’, and ops, meaning ‘face’, these short globular plants grow as two fleshy leaves fused at the base. Growing together in clumps, their flat pebble-like appearance allows them to blend in with the surrounding rocks to avoid being eaten. Lithops are a highly desirable plant for collectors all around the world, and the multitude of different colours and patterns which they exhibit correlates with the various substrates in which they are camouflaged: blue-grey leaves blend in among quartz rocks, mottled green and black leaves are cryptic in gravel, orange and brown patterns help them remain hidden in sandy ground, and near-white growths keep the plants concealed in saltpans. It is often only during the period of summer rainfall that these plants become apparent in their landscape, when each pair of tender leaves splits apart to produce a single delicate flower.

Some of the larger desert plants also use forms of disguise to keep themselves hidden from herbivores, and a handful of cactus species ingeniously avoid being eaten by grazing mammals by looking like something that may have already been eaten – they resemble droppings. Herds of hungry llama-like guanacos (Lama guanicoe) together with javelina (Pecari tajacu) and an introduced population of goats are a menace to the plants of Chile, stripping their succulent stems for a quick meal. But one species of cactus called Copiapoa laui has evolved a way to make it look especially unappetising, by growing in dark patches of spineless, flat, diskette growths roughly a centimetre in diameter, which to the untrained eye seem to be a scattering of mammal dung. Another plant which uses the same method is a species of cactus called Lophophora williamsii found in the Chihuahuan Desert. Its 7-centimetre-wide stems often become flattened to only a few centimetres and as they grow and overlap each other they take on the distinct shape of a large pile of mammal droppings. However, because of its size L. williamsii has had to evolve a more powerful defence. Its squat stems are full of highly toxic chemicals, containing upwards of 60 different alkaloids, and as a result it is extremely bitter and distasteful to herbivores. This dissuades any animals from attempting to eat it. Should any human ingest the plant’s powerful toxins, though, the effect is more extreme, as the principal alkaloid is mescaline, a powerful psychoactive drug. The potent hallucinogenic properties of L. williamsii and its relatives appear to have been known for thousands of years, with archaeological evidence from Texas suggesting that it was used by the native people of America’s southwest as far back as the middle of the Archaic period, around 5200 years ago. Referring to the plant as peyote, a derivative of the Nahuatl word peyotel or peiotl, Native American tribes heralded its curative properties for diabetes, fever and the relief of pain during childbirth. The Huichol people of Mexico regard the ‘sacred peyote’ as a gift from god, and the overwhelming spiritual images evoked by the hallucinogenic effects of the cactus have had a profound effect on their religious beliefs and philosophies. In more recent history the powerful narcotic effects of L. williamsii’s chemicals are accredited as having influenced a whole generation of musicians, writers and poets, ranging from Allen Ginsberg to Aldous Huxley.

The most obvious method of protection used by cacti and succulents living in arid environments is their impenetrable shield of spikes, spines, thorns or prickles. These sharp woody growths have evolved as a result of adaptations towards smaller leaves to reduce water loss – as smaller leaves lose less water than larger ones, smaller leaves were selected for over millions of years of evolution, and as they diminished they also became sharp and pointy. Gradually they became saturated with calcium carbonate and pectin, which made them rigid and tough. Eventually the thickened stems of cacti took on the photosynthetic role of their leaves, allowing the leaves to become modified into the needle-sharp spines we see today. Evidence for this gradual transition can be found by looking at the earliest stages of developing cacti spines, which look nearly identical to the earliest developmental stages of leaves on other plants. They are produced at the base of the axillary bud’s shoot apical meristem. However, apart from their shared origins cacti spikes bear no resemblance to leaves whatsoever: they have no xylem, no phloem, no stomata and no chloroplast, and when they are mature their cells are dead.

Desert survivor
While Welwitschia mirabilis rarely look very healthy, dried and twisted on the desert floor, they are known to live for up to 2000 years.
© RBG Kew

There are a great number of spiky desert plants which thrive in arid habitats all across Africa, but the spikes of these plants have evolved differently. For example, Euphorbia neohumbertii from Madagascar exhibits a body very similar to the cacti of America as a result of evolving under similar environmental conditions – it has a tall, water-storing green body, covered in spikes. To the untrained eye this plant looks just like a cactus. But the spikes of Euphorbia and their relatives have evolved from modified shoots, not leaves, and where cactus spikes are smooth and grow in unbranched clusters, the spikes of Euphorbia only occur singularly and never in clusters, often branching with tiny scale-like leaves. Where a cactus is essentially a water-storing stem with no leaves, the doppelganger Euphorbia neohumbertii is a giant water-storing leaf with nearly no stem. The amazingly similar appearances of these unrelated plants, which have evolved independently thousands of kilometres apart, is the result of a phenomenon called convergent evolution, and it is seen in a number of other dry-zone plants which have adapted to similar environmental constraints in separate habitats. For example, aloes from Africa and agaves from South America have both evolved tough fibrous spear-like leaves arranged in a rosette. Aizoaceae from southern Africa and Ariocarpus cacti from Mexico have both evolved to bury their small succulent bodies in the substrate to keep cool. Similarly, Euphorbia obesa from the Eastern Cape and Astrophytum asterias from Texas have evolved seemingly identical octagonal/round bodies.

Drinking fog
This species of Astrophytum or star cactus has a rough waxy surface which captures minute droplets of airborne water which are then channelled by its ridges to its base.
© SuperStock

In whatever way they are produced, the protective spikes of dry-zone plants appear in a dizzying array of forms, appearing in innumerable variations of shape, number and function. Some cacti, such as Mammillaria poselgeri, known as the fishhook cactus, have long hook-shaped spines which are thought to ‘educate’ herbivores not to attempt to eat the plant after an initial ‘hooking’ by the plant’s spines. The same effect is achieved by the pairs of opposite-facing thorns of Acacia mellifera, which catch the fur of grazing animals that get too close to it. Some of the longest spikes can grow up to 25 centimetres long, as seen on Trichocereus chiloensis from Chile, and the Tephrocactus paediophilus has razor-sharp, flimsy spines two or three times the length of the plant. More often spines grow to just a few centimetres in length, protruding in crown-like bundles from the plant’s stem. These bundles can be long and thin or can produce stocky, broad clumps of spikes. Some more primitive cacti have spines which cover their flowers as an added defence, and other cacti have adapted their spikes to have an added advantage of camouflage such as Sclerocactus papyracanthus or grama grass cactus, whose sharp 5-centimetre-long spines have evolved to look like dried brown grass to conceal its succulent green body from herbivores. Cacti also have highly effective, almost microscopic, spines called glochids, commonly seen on species of Opuntia or prickly pears. At microscopic levels these tiny hairs can be seen to have barbed tips, making them particularly tricky to remove.

While the drought-resistant plants of the world’s arid landscapes are masters at retaining their water and keeping away unwanted animals, there are times when these unmovable desert dwellers must leave themselves susceptible to desiccation to produce the flowers which attract their pollinators. For all xerophytes the production of flowers is a costly venture, as flowers are relatively delicate structures prone to drying up and losing water in extreme heat. Where the flowering plants of the temperate and tropical regions of the world can produce blooms all year round, those in the dry zone must be more conservative with their resources. Instead of having long flowering periods, it is far more economical for them to wait and only produce their flowers when conditions are optimum. This ability to wait for the right conditions enables many plants to survive in the dry zone.

In true deserts like the Tucson Desert the opportune time for flowering can be as infrequent as every 10 years, and only after substantial winter rains and an amiable summer climate is the environment transformed by a sea of floral colour. When these conditions arise, carpets of yellow and orange Californian poppies (Eschscholzia californica), spearhead-like lilac lupines and the deep-purple heads of owl clover (Castilleja exserta) bring a stunning spectrum of colour to the desert. The same is seen every April in the semi-arid lowland steppe of western Kazakhstan, where a small amount of precipitation sees the dried brown ground turn a lush green, as the landscape is filled with a host of tulips in a dazzling array of yellows, purples, whites and reds. But after only a few months of colour, these pastures are once again turned barren by dry winds.

Hylocereus undatus, the ‘queen of the night’ cactus from Central America, flowers just one night a year. The shrivelled appearance of this vining cactus is a tangled mass of succulent tubes, each a few centimetres in diameter, that tumble over rocks and hang over the branches of other plants on which it has become established. It is, however, perhaps the most exquisite member of the cactus family. As the temperatures in the tropical deciduous forests where it grows can reach a crippling 40°C in the summer, this cactus is largely inactive during the day. In late spring each year its sprawling vines begin to produce small protrusions which grow in length over a number of weeks, and by midsummer these have become 20-centimetre-long spear-like green buds. The plant then sits idle, remaining poised for a full moon, when it springs to life in a display of astounding beauty. As the sun begins to set and the light of the moon emerges, the flower’s buds begin to move, slowly extending the sticky ends of its yellow stamen from the tips of its buds, closely followed by a brush of pollen-covered anthers. Over the next couple of hours the fleshy green clasp-like tepals and bracts begin to peel back as the bud slowly opens to reveal a head of creamy white petals, unfurling into a glorious flower. As the flowers open they emit a sweet fragrance, attracting hawk moths and nectarivorous bats that fly up the odour plume. As they get closer to the plant the magnificent 30-centimetre-wide flowers appear to glow in the light of the moon, drawing its pollinators to feed on its nectar. After only a few hours, the flowers of this night-blooming cactus begin to close again, and by the time the morning sunlight emerges, the flowers have already wilted and died. This is all part of the plant’s survival strategy, however, as the closed petals form a protective shield around the newly fertilised seeds, locking vital moisture inside. The queen of the night’s approach to producing flowers that only last one night is a tactic which has proved highly successful.

Desert colour
The springtime wildflower bloom of Antelope Valley Californian Poppy Reserve is one of the most spectacular desert sights.
© SuperStock

Agave americana is more commonly known as the century plant, as those who first discovered it in its native habitat in Mexico believed it to flower only once every 100 years. The name has stuck, even though it is now known that it does not flower as infrequently as that. The powdery blue leaves of this massive succulent are edged with serrated tooth-like structures which clump together to form a wide rosette reaching 2 metres high and 4 metres wide. Like many plants that have evolved to survive the harsh conditions of the dry zone, A. americana is very slow-growing, and as it grows it builds up food stores of sugar and starch in its tough body. As the plants that share its habitat go about their regular cycles of flowering and dying, the agave remains unchanged. Over a passage of 30 years or more, the agave will slowly expand as it gradually builds its internal store of energy.

Queen of the night
The dinner-plate-sized flowers of this plant are beautifully fragrant but only bloom for one night a year.
© RBG Kew

Eventually the agave will have amassed enough energy over its long life, sometimes as long as 60 years, at which point it begins to channel its store of carbohydrates into developing a flower spike. From a life of veritable stasis the plant’s metabolism kicks into overdrive, and as this spike emerges out of the centre of its broad leaves it reaches a growth rate of up to 25 centimetres a day. Resembling a giant asparagus stalk, the agave’s flower spike rockets up to a height of 8 metres, where it then splits to produce a metre-wide branched structure covered in dense clusters of buds. Over a period of two weeks the flower spike will produce tens of hundreds of strongly scented pale yellow flowers, which act as a beacon to night-foraging bats. Each bud of the agave’s bloom is oozing with copious amounts of nectar, which is produced using the plant’s lifetime store of energy, which in turn ensures it is visited by a sufficient number of pollinators. However, it is only after A. americana has flowered that the importance of its patience of many decades becomes clear. When its flowering head has produced its many thousands of wind-dispersed seeds and its flower stalk collapses, so too the rest of the plant begins to shrivel. As quickly as the plant shot out its towering flower stalk to spread its genes, its metabolism slows down and the plant begins to wither and die. The plant literally flowers itself to death, yet relies on at least one of its thousands of seeds to successfully germinate.

Pushed to the Extreme

For the many varieties of plants that have evolved specialist adaptations to live and reproduce in the unforgiving conditions of the planet’s dry zones, it is a result of hundreds of thousands of years of natural selection that has finely tuned them to be able to do so. As the conditions of these habitats became more extreme for plants to survive there, so the bodies of plants became more extreme. The actions of humankind, however, have had a historical propensity to upset the balance of nature, and as a result many plants now have an ‘anthropogenic extreme’ subjected upon them.

Such plants are perhaps worst affected on islands – areas of limited space in which the ever-growing human population is forced to strip the native vegetation for food and resources, facilitating further population growth, which in turn requires even more resources. Island species are especially vulnerable in that they have evolved in line with the very specific parameters of their unique habitat, which can easily be upset by sudden change, throwing the island ecosystem out of equilibrium. When island species are threatened by changes in their immediate habitat, they have nowhere to go. For millions of years the dodo lived a happy existence on the island of Mauritius in the Indian Ocean; its population was kept in check by the natural forces of competition and disease. However, with the arrival of human settlers in the early 1600s, the birds were unable to escape the cats and dogs brought to the island, and they were unable to protect their ground nests from the introduced pigs and macaques. The dodo was soon wiped out. This is the age-old tale which currently threatens the exotic biodiversity of Madagascar and Borneo, the islands of Indonesia and the Philippines, as well as similar areas of rich diversity across the globe. From rainforests to mountain ranges, evidence suggests that species across the world are disappearing in variety and number faster than at any time in the Earth’s recent history. But fortunately there are those who have dedicated their lives to the meticulous study of the plants and animals of almost every environment on Earth, providing an invaluable resource by which we can monitor the state of our planet.

Spikes and spines
As well as protecting the plant against herbivores, these varied structures also play an important role in temperature regulation and collecting moisture.
© Will Benson

One such resource is the Herbarium of the Royal Botanic Gardens at Kew. Since its inception as a botanical garden in 1759 Kew has been at the forefront of plant science and discovery, and the Herbarium is a testament to this legacy. Founded in 1853, the building was intended to hold the dried collections of preserved plant and fungus specimens that had been collected by botanists and horticulturalists, from both Britain and overseas. With the rapid expansion of the British Empire during the nineteenth century, the Herbarium’s collections rapidly grew. Today staff process over 50,000 specimens a year that come from across the globe, with the collection as a whole now totalling over seven million individual specimens. The Herbarium as it stands is the largest repository of botanical data in the world, and more importantly, it is a key weapon in safeguarding the future of plants on our planet. This unique databank of preserved specimens along with detailed descriptions of their habitat and relative abundance plays a vital role in monitoring the global health of plants.

The Herbarium at Kew
The world’s largest repository of botanical data, and a key resource in plant conservation.
© RBG Kew

Exactly how important this collection is as a tool for global conservation is exemplified in the case of one particular plant from the island of Rodrigues in the Mascarene Archipelago, called Ramosmania rodriguesii or café marron, a wild member of the coffee family. Through their many thousands of years of geographic isolation the Mascarene islands, including Mauritius and Rodrigues, have evolved many unique and fascinating species of flora and fauna. However, since the arrival of European settlers in 1638 their endemic flora has slowly diminished in the face of habitat destruction and the arrival of introduced species. On the smallest of the islands, Rodrigues, eight species of plant are already known to have gone extinct, and of 38 remaining endemic species 21 are listed as being endangered, including one particularly enigmatic species called the café marron. This tree, 2–4 metres tall, with lush waxy leaves and pentamerous white flowers, was first discovered in 1874, when its ‘type’ or primary specimen was sent to the Herbarium at Kew and it was given its Latin name, preserved and catalogued. Apart from a rough drawing of the plant made by a European visitor to the island in 1877, little more was heard or seen of this species for many decades, and as the populations of introduced pigs and goats increased, the plant’s population began to dwindle. By the mid twentieth century café marron was assumed extinct from the island. But then in 1979 a young boy named Hedley Manan, from a school party that had been encouraged to look for a number of rare plants on the island, found a lone specimen of what appeared to be the plant from the 1877 sketch. A cutting was taken from the shrub and sent to Kew, where it was compared with the preserved specimen in the Herbarium – and indeed it was the very same species. This was great news for botanists and conservationists alike as it spelled hope for the chances of re-establishing café marron on the island. However, with the fate of the whole species resting in this one lone plant, scientists and horticulturalists knew they would need to act quickly. With help from the IUCN and the Mauritian Forestry Service a further cutting was sent to Kew, where it was successfully propagated in the Temperate Nursery, producing dozens of genetically identical new plants.

The living dead
Café marron’s story of survival is an example of just how important centres like Kew are for preserving our planet’s species.
© RBG Kew

Back in the UK the cuttings grew rapidly, and soon the plants began to produce white flowers. But despite numerous attempts it proved impossible to pollinate the flowers and therefore the plants couldn’t produce any seeds. The horticulturalists at Kew began to assume the worst, with all the evidence suggesting that the last wild plant from which the cuttings had been taken must be sterile. As only one living plant was known, the horticulturalists had no other suitable specimens to compare it to, and as a result it was impossible for them to assess whether their plant was male or female, or indeed both, or if it was mutated in some way that might explain why it could not be pollinated. For many years Kew continued to propagate cuttings from the one remaining plant in an attempt to produce seeds, to no avail. In 2001 a handful of cuttings were repatriated to Rodrigues to be planted in a fenced-off area, but ultimately their population was doomed if they were unable to naturally reproduce. Before long the species acquired the name ‘the living dead’, as even though its cuttings could be planted and re-planted to produce an indefinite number of identical clones, without cross-pollination the gene pool of the species would be so small that a single bacterial or viral disease could wipe out the entire population. As well as the scientific and botanical struggle to unravel the mysteries of the café marron in England, the lone survivor and its re-planted clones back in Rodrigues were dealing with further persecutions still – the attention that these meagre cuttings were receiving in their natural habitat from conservationists led locals on the island to believe that the plant possessed curative properties. Fences were erected to protect the plants, but people soon cut through to cut off parts of the plants to use in hangover cures or to treat venereal diseases. Eventually three layers of 3-metre-high fences were put in place to keep the last café marron safe, and there it remained, safely entombed, waiting for someone to provide the key to its survival.

Back at Kew the continuous blooming of the cloned plants in the nurseries inspired one horticulturalist named Carlos Magdalena to try any method that could conceivably work to pollinate one of the flowers. Together with his supervisor Viswambharan Sarasan, Carlos tried various methods to cross-pollinate the flowers of the cloned plants, and eventually they came to amputating the stigma from one flower and directly transferring its pollen to another flower. Nine hundred and ninety-nine of these attempts proved fruitless, but in the summer of 2003 one flower on one particular plant showed a small swelling of its ovary, and within a few weeks this plant produced a fruit containing the crucial seeds of a new generation of café marron. Upon ripening, the seeds were rushed to Kew’s conservation biotechnology laboratory to be planted. Unfortunately their worst fears were realised – the seeds had failed to germinate. But even though no new plants had been acquired, the production of the fruit was the evidence that the team needed to prove that the last café marron wasn’t sterile. However, as only one in a thousand ‘cut and stick’ trials was successful, Carlos believed that some other factor must have caused this one plant to fruit when it did. On analysing every aspect of the conditions of the plant he recalled that the fertilisation had occurred amidst one of the hottest heatwaves that southern England had experienced in years, and at a time when the shades of the nursery roof had been broken, preventing them from closing. He deduced that the exposure to excess heat and light could have triggered the plant to fruit. Carlos began moving café marron plants into areas of exposed sunlight along the heating pipes that skirt the tropical nursery, and lo and behold many more plants began to fruit. As before, seeds from the fruit were rushed to the propagation units of Kew’s conservation biotechnology laboratory, and a month later four out of five seeds had put out roots and shoots – the long-awaited lifeline for the next generation of café marron.

Carlos monitored the new plants as they began to grow, but quickly spotted that the plants that began to sprout did not have the lush, oval waxy leaves of the adult plant and its cuttings, but instead put out long thin brown leaves which looked almost dead. He was trying to propagate saplings from an elegant broad-leaved tropical tree and the plants he saw before him resembled something more like an ugly scrubby shrub. As he watched the saplings growing over a number of weeks, he saw their tattered brown leaves get longer and longer as the plants got taller, but then miraculously at around a metre in height their leaves began to change. Slowly their thin leaves plumped out, and astonishingly the adult plant revealed itself as the lush green tree easily recognisable as café marron. This amazing morphogenesis is thought to be a result of the plant’s historical relationship with the native fauna of its island, the Rodrigues giant tortoise and a large bird called the solitaire, both of which are now extinct. As tortoises have very poor eyesight they rely on finding large green leaves to eat that are highly visible in their habitat. The thin brown leaves of the young café marron would therefore go unnoticed by a hungry tortoise, and it is only by the time they are well out of reach of the herbivore’s straining neck that they produce their thick green, veritably appetising leaves. This strategy, called heterophylly, is seen also in the raintree (Lonchocarpus capassa) which grows in the savannahs and woodlands of southern Africa. Its leaves look grey and diseased to deter herbivorous antelope when it is at their head height as a small shrub, and only later, when it is a mature tree many metres high, does it produce lush green leaves.

Today the new café marron plants are producing their own flowers and their own seeds. From at first containing just five seeds, fruit are now being produced containing up to 85 seeds each, and slowly but surely a new population of genetically varied café marron is being re-established on Rodrigues, with the hope that once again it can flourish in its natural habitat. On the one hand this plant’s adaptations to its environment remind us of the unyielding and inherent ability of plants to survive. On the other hand, the story of the café marron outlines the important work that continues to be carried out by botanical gardens and research institutes across the world, to help bolster populations and maintain the vital biodiversity of species on Earth.