Plants that Kill to Survive

Forest and Bird, Issue 305, 1 August 2002, Page 14

Plants that Kill

to Survive

BRUCE SALMON

introduces and photographs

the carnivorous plants of New Zealand:

ome of New Zealand’s most remarkable plants eat insects to balance their diets. Our carnivorous plants exhibit an extraordinary behaviour where plants have turned the tables on animals in the order of the food chain. Carnivorous plants are among some of the most bizarre and fascinating life forms ever produced by the vegetable kingdom and there are many myths surrounding them. Extravagant reports were widespread in the 1800s and early 1900s as explorers were discovering the botanical treasures of new and exotic lands. Speculative articles about man-eating trees were even published in reputable magazines such as Scientific American. These reports captured the imagination of the public and led to many books and films, including the well-known The Day of the Triffids. Fortunately for us it is only insects that are at any real risk from these plants. I can still remember the day I found my first native carnivorous plant in the wild — a sundew, Drosera auriculata. It was growing

on a steep clay bank in the hills near Clevedon south of Auckland. Its pendulous leaves bobbed up and down in the early spring breeze and its dewdrops sparkled like a thousand diamonds in the sunlight. I was immediately hooked. This was surely the most fantastic plant I had ever seen. Since then I have spent more than 15 years travelling the length and breadth of New Zealand in search of them, from the swamps of the Far North to the mountain tops of Fiordland, culminating in my recent book Carnivorous Plants of New Zealand.

Not all plants which can catch and kill insects are carnivorous. Many plants employ a whole host of deterrent and trapping devices in their continual war with the ravaging insect hordes, but they don’t eat what they catch. Commonly known plants that have insect-trapping defence mechanisms (usually sticky hairs) include tomato, potato, petunia, rhododendron, and our own notorious bird-catching tree parapara with its sticky seed pods. By definition a carnivorous plant is flesheating. It must attract prey, capture and retain it. The most important of these

criteria is that a true carnivorous plant must be able to absorb substances derived from its prey, to provide elements essential for its healthy growth, that are otherwise missing from its environment.

To aid digestion carnivorous plants may use certain fungi and bacteria to decompose their prey or degrade the insect’s exoskeleton so that the plant’s enzymes can then go to work on the soft internal organs. Bacteria are associated with all carnivorous plants and their action may provide the only means of digestion for some. The digested by-products are then absorbed by the plant and used for its growth and development. Carnivorous plants require impoverished soils, a plentiful supply of water for at least part of the year, and exposure to high light levels. Poor soils are usually deficient in nitrogen, potassium and phosphorus and are often acidic in nature, yet these essential elements are required for healthy plant growth. Carnivorous plants obtain them from the bodies of the insects they capture. This gives them a distinct advantage over most normal plants which struggle to survive in such environments. It is thought that insect-eating by plants is possibly a response to the nutrient-poor or saturated soils in which they grow. The high light requirement is necessary to support their carnivorous processes. The environmental conditions the plants favour are often associated with bogs and a great proportion of carnivorous plants are in fact bog plants. Native carnivorous plants occur throughout the country from coastal fresh-water sand dune marshes right up to alpine tarns

high in the mountains. Other favoured habitats include lakes, bogs, seasonally dry scrubland, slips, clay banks and exposed ridge tops. Sadly, many of these habitats have been contaminated with fertiliser runoff from farming activities making them unsuitable for carnivorous plants. If you should decide to venture into these habitats special care should be taken as many are easily damaged by trampling and can take a long time to recover. Some bogs can also be treacherous for the inexperienced. f the 600-odd species of carnivorous plants found worldwide New Zealand has only 12. They are of two kinds, also the two most common worldwide — the sundews, with seven species, and the bladderworts, with five.

Sundews (Drosera)

All sundews use an active fly-paper method to trap crawling and flying insects. Glistening dew is secreted from the tips of numerous hairs covering the leaf’s surface which may attract insects to their doom by ultraviolet light patterns. Once ensnared the insect’s struggling stimulates these hairs (or tentacles as they are more commonly known). These tentacles bend slowly inwards, thus securing the hapless insect to the leaf’s centre where the digestive glands

are concentrated. These glands secrete a cocktail of digestive juices that break down the insect’s soft tissues which are then absorbed by the leaf. To conserve valuable energy the plant only uses as many tentacles as is necessary to secure and digest its prey. For tiny insects only a single tentacle may be employed, whereas for larger prey more than one leaf may be employed. After several days the insect is digested and the tentacles return to their original positions. All that is left is the insect’s rigid exoskeleton which will be washed away with the rains. Over a period

of time the leaf becomes inoperative and dies, to be replaced by another. An organ housed in the tentacle’s bulbous tip is very sensitive to touch and chemical stimuli. Experiments performed by Charles Darwin proved that a piece of human hair weighing 0.0008 milligrams could cause the tentacle stalk to move. The more intense and prolonged the stimulation, the faster a tentacle will bend, often within two or three seconds. The object in contact with the

tentacle tip must either stimulate it with continuous movement or cause a chemical input before the tentacle will react. This prevents the plant from wasting its energy trapping bits of wind-blown leaf or dirt. New Zealand has five lowland and two alpine species of sundew, also known as wahu in Maori. Almost all are shared with Australia except for Drosera stenopetala which occurs nowhere else. In fact its closest relative comes from southern Chile (another link from the ancient supercontinent of Gondwana to which we were once connected). Of the lowland species, three are summer growers and include a pygmy species Drosera pygmaea only one or two centimetres across. Drosera spatulata is a flat-rosetted species up to 3 centimetres across. The largest is Drosera binata with forked leaves which may reach 30 centimetres tall. They occur throughout New Zealand and prefer habitats that are

permanently wet, such as peat bogs, seepages, heathland and lake shores. The other two Drosera species, Drosera auriculata and Drosera peltata are winter growers. They have erect stems with the trapping leaves scattered along them and tubers with which to survive dormant through a dry summer. They are often found growing in moss pads on clay banks, heathland and semi-dry manuka scrubland in the North Island and northern tip of the South Island. Our two alpine species, Drosera arcturi and Drosera stenopetala, are also summer growers breaking dormancy in spring, having a brief feeding season and then dying back to a tight bud to survive the winter snow. They can be found in alpine bogs, seepages and around the edges of small pools and tarns, usually above the tree line from Mt Ruapehu southwards. The flowers of our native sundews are not at all showy except for Drosera binata which may have several flowers of two centimetres diameter open at once. Most are small, white and inconspicuous.

Bladderworts (Utricularia)

Bladderworts have developed a unique vacuum-trapping mechanism that is the fastest of all the carnivorous plants. It is much faster even than the well-known snapping jaws of the Venus flytrap. Tiny bladder-like traps only one or two millimetres in diameter arise from the plant’s network of slender white stems which spread just below the surface. Even though their traps are relatively minute they are still very efficient, catching many soil

and water-borne organisms such as nematodes, crustaceans, water fleas and even mosquito larvae. Bladders have a one-way trap-door situated at one end with trigger hairs on the outer surface. The door is sealed with mucus, which can also attract prey, and has several antennae around its periphery that help guide prey to the trap entrance. When triggered, these hairs break the seal on the door and the prey is instantaneously sucked in with the door snapping shut behind it. All this happens in as little as 30 milliseconds! The excess water ingested with the prey is then slowly excreted and the trap is reset. Movement by the prey stimulates the release of digestive enzymes from glands on the inner walls. Unlike sundews, bladderwort traps cannot expel the remains of their prey, so the traps cease to function once they are full. New Zealand has two terrestrial and three aquatic species of bladderwort. Most also occur in Australia but one species, Utricularia delicatula, exists nowhere else. In the terrestrial species it is only their leaves that are seen above ground with the traps arising from the fine stems extending just below the surface. Their small leaves are often overlooked and it is only when they flower that they are noticed. The flowers are about one centimetre across and colours

include white, lavender and purple. Both our terrestrial bladderworts, Utricularia delicatula and Utricularia dichotoma, favour sunny locations in permanently wet open ground that is poor in nutrients. They are often found in acid bogs, seepages and lake margins growing in anything from pure sand to peat or sloppy muck. Utricularia delicatula occurs in the northern half of the North Island whereas Utricularia dichotoma occurs on both the two main islands and Stewart Island where it maintains the southern limit for the genus. The aquatic species are usually floating; their long branching stems support numerous divided leaves from which the bladder traps arise. Two species Utricularia australis and Utricularia geminiscapa look very much like water milfoil. The third species Utricularia gibba has very slender stems that intertwine to form a thick mat that may either float or be affixed to the lake bed. Most have yellow flowers about one centimetre wide that are borne in summer, making them easy to spot at a distance. Our aquatic bladderworts are found mainly in still acidic backwaters, such as peaty pools and shallow lakes, often amongst emergent rushes in the northern half of the North Island.

BRUCE SALMON is author of Carnivorous Plants of New Zealand and may be contacted at 13 Rothery Road, Manurewa, Auckland.