Kingdom of Plants: A Journey Through Their Evolution. Will Benson
vines
The vascular system of lianas has evolved to transport water hundreds of metres through the canopy.
© RBG Kew
Lianas take up water and nutrients for the plant from the forest floor, but in order for them to reach up to the top of the canopy they must be able to transport their nutrients through their elongated stems – as far as 900 metres in the most extreme examples. While the evolution of lianas has resulted in one of the most advanced water transport systems of any plant, there is a group of specialised plants that have found a way of living high up in the architecture of the forest trees without the need for long roots. These plants are known as epiphytes, or air plants, as they do not have roots which grow into the soil, but instead have short roots which can absorb nutrients and water from organic matter that accumulates on branches high up in the forest canopy. As these plants must absorb all of their water from the air, they can only thrive in very humid environments, and in the high-altitude montane forests of the tropics trees can become covered in epiphytic plants. Epiphytes include the mosses and lichens of temperate forests, but also a multitude of more complex plants such as orchids, ferns and some tropical cacti. However, there is one group of plants that stands out as the true masters of arboreal life, the tank bromeliads. Relatives of the pineapple, tank bromeliads live attached to the branches of trees in the rainforests of South America, where they display an amazing variety of shape and colour, from broad, green fleshy-leaved plants, to small, delicate purple and red structures. Their broad leaves are arranged into a basket-shaped rosette which acts to channel the rainwater that trickles through the forest canopy into a central reservoir, and the bases of these leaves are packed so tightly together that they are able to create a watertight tank in which the water can gather. In the largest species of tank bromeliad as much as 50 litres of water can be held between the plant’s leaves, and a study conducted in Puerto Rico found that in just a 1-hectare area of rainforest as much as 50,000 litres of water can be stored, which would fill a small swimming pool.
Tank bromeliads
These plants provide arborial homes for a whole host of animal life.
© RBG Kew
The little elevated lakes that gather in the bases of bromeliads make perfect homes for a handful of species looking to escape the perils of the forest floor. Insects such as mosquitoes lay their eggs in the pools of water, and flatworms find shelter among the leaves. With this abundance of insect life comes a menagerie of larger animals that visit the pools to munch on the invertebrate feast. Inch-long salamanders come to feast in the relative safety of the plant, and in some bromeliads in Jamaica small crabs have been found to dwell, territorially defending their plant against lizards and millipedes. Tiny poison dart frogs are perhaps some of the best-known lodgers, with some species spending their entire lives, from tadpole to adult, inside the seclusion of the bromeliad’s tank. A recent count of the different animals that live in the bromeliads of Ecuador found an astonishing 300 different animal species that made these plants their home. However, the bromeliad does not just provide a home for these animals to be of service, rather it accommodates them so they can provide it with food. The droppings of the frogs and salamanders, which contain the digested bodies of the insects that gather in the pool, accumulate in the water and can be absorbed by the plant as a vital source of nitrogen-rich food.
Back on the forest floor plants grow in a more conventional way, rooted to the ground. Although these plants only receive around 2 per cent of the glorious sunlight available at the top of the canopy, they have the advantage of growing directly on the nutritious layer of organic material created by an unseen army of bacteria and fungi. Species here are able to spread across the forest floor in a dense carpet of growth, employing a number of adaptations which allow them to thrive in the forest understorey. One such strategy is seen in the stripy-leafed Tradescantia zebrina from the forests of southern Mexico, which has purple undersides to its leaves created by a pigment called anthocyanin. As light passes through the green photosynthetic tops of the leaves the purple cells underneath act like a mirror and bounce the light back up to ensure that the maximum energy is absorbed by the plant’s chlorophyll. Other plants use size to their advantage to capture as much light as is physically available. One of the most successful plants at using this strategy is the giant taro plant, Alocasia robusta, which has the largest undivided surface area of any leaf on the planet, growing to over 3 metres in length and over 2 metres in width. Its huge glossy leaves thrive in the understorey of the tropical forests of Asia, where they fan outwards in order to gather light throughout as much of the day as possible. Another more subtle mechanism used by plants to ensure they can gather as much light as possible is found in a small purple-leafed shade-dwelling species called Oxalis oregana, from the redwood forests of western USA. At the top of its short 15 cm stems it has triplets of heart-shaped green leaves which are able to move in order to track the sparse sunlight as it shines through the canopy above. However, as Oxalis has adapted to photosynthesise in such low levels of light, strong sunlight can actually be damaging to its cells. Consequently, should a beam of sunlight break through the canopy directly onto its leaves, in just 6 seconds the plant can tilt its leaves to a vertical angle and escape the light.
Another group of prolific plants that thrive in forests are the palms, notable for their economic importance to us. Found growing both as tall trees with mighty crowns poking through the forest canopy and in short, spiky clumps at ground level, palms can live in habitats ranging from the desert islands of the hot tropics to the milder Mediterranean climes of the subtropics. They are instantly recognisable by their distinct leaves. Resembling thick, green feathers and broad, fan-like paddles, their leaves give them a surface for absorbing energy from the sun, and their deep ridges channel rainwater away from their surfaces. Various animals also rely on palms for food and shelter: birds such as palm-nut vultures and macaws flock to the plants to eat their fleshy fruits, and small, ground-dwelling mammals like the Asian palm civet (Paradoxurus hermaphroditus) forage for fruits about their bases. Madagascar has the highest diversity of palms anywhere on the planet, and because of the island’s lack of herbivores the leaves of its palms lack the chemical defences and spines found elsewhere. Some of the island’s palms have leaves that extend for no more than a few centimetres, but in the extreme example of the raffia palm (Raphia farinifera), its leaves can hang down from the crown a massive 24 metres – the longest known leaf of any plant, roughly the height of a seven-storey building. The largest seed of any known plant comes from a palm too, called the coco-de-mer or double coconut, and the largest known inflorescence comes from a species of palm called Corypha umbraculifera. The eighteenth-century Swedish naturalist and father of taxonomy, Carl Linnaeus, was so taken by palms that he labelled them as Principes, the order of the Princes.
Leaf architecture
Leaves are the food-making factories of green plants, and come in a variety of shapes and sizes.
© Will Benson
Palms today provide humans with an array of useful materials and foods, and after grasses and legumes they are the most economically important plants on the planet. Nearly every part of the palm plant can be used for food: the sap is commonly boiled to create a sugary food called jaggery, and the oils from the flower are tapped to make a fresh drink or fermented and distilled to create a number of potent alcoholic beverages, such as the east Asian liquor arak. The sweet tips of the fresh leaf growth make a sweet salad, and the starch from the fibres of the trunk can be harvested to make a nutritious food called sago. As well as their multitude of edible uses, palms provide a variety of practical materials, both locally to where they are grown and across the world. Their wood is used to make buildings and furniture, as well as ropes, clothing and fibres, and the oil from their fruits can be turned into waxes, fuel and cheap cooking oil. Sadly, the oil produced from palms is in such high demand that great swathes of diverse tropical rainforest are being felled in Southeast Asia to be replaced with vast seas of oil-palm plantations.
Mighty bamboo