Learn about parasitic plants
They take without giving anything in return to their hosts, sometimes even killing them
PARASITIC plants can be described as the thieves of the plant world, and rightly so. These plants take without asking and without returning anything to their unwilling host plants.
WHAT IS A PARASITE?
A parasite – which can be a plant or an animal – is an organism that lives and feeds off another species, harming it in the process.
The species (plant or animal) on which the parasite lives, is called the host. The host weakens, especially in its ability to reproduce, while the parasite grows stronger and its ability to survive improves. So while the parasite feeds off the host, it doesn’t necessarily kill it as it needs the host to survive.
Parasitic plants have a special root-like structure called a haustorium, which is used to penetrate the host to extract nutrients and water.
TYPES
Parasitic plants account for just more than 1% of the world’s flowering plants. There are more than 4 000 species and about 450 species in South Africa. They’re characterised as follows: HEMIPARASITES These are mainly dependent on the host for water and nutrients (hemi is Greek for “half”). But these “plant thieves” also produce their own nutrients through photosynthesis as they have chlorophyll so their leaves can be green.
You might have heard of the Christmas tradition of kissing under the mistletoe. This plant is a hemiparasite that steals its host plant’s nutrients. The botanical name of mistletoe, Phoradendron, means “tree thief” in Greek. The Western Australian Christmas tree (Nuytsia floribunda) is another hemiparasite.
HOLOPARASITES These parasites are completely dependent on the host (holo means “wholly” or “completely” in Greek). These parasites can’t produce their own nutrients and can’t photosynthesise. They sink their roots into the branches and bark of the host tree and extract carbohydrates, water and minerals.
The members of this group, for example, are fast growers and aren’t particular about their victims. Some holoparasites live alongside the host without killing it, while others take increasing amounts of nutrients the host can’t provide.
The host becomes vulnerable to illness and insects and after a while it dies but by that time the parasite has reproduced. Dodder (Cuscuta) is an example of a holoparasite. OBLIGATE PARASITES These plants can’t reproduce without help from their hosts. Branched broomrape (Orobanche ramosa) is
an example that frequently attacks crops such as tomatoes and tobacco.
FACULTATIVE PARASITES These plants might resort to parasitic activity, but don’t rely completely on hosts for the completion of their life cycle and can survive without a host. The snow plant (Sarcodes sanguinea) is an example. ROOT PARASITES They attach to the host’s roots system. The flowers of some root parasites have beautiful bright colours and a velvety texture to attract insects for pollination. A local example is jackal food (Hydnora africana) that looks like a fungus except it has flowers.
Some of these parasites bear exceptionally large flowers with strange fragrances and shapes. The parasitic flowering plant stinking corpse lily (Rafflesia Arnoldii) has the largest individual flower on Earth – measuring 1m in diameter and weighing up to 1kg. It’s indigenous to the rainforests of Sumatra and Borneo.
It gets its name for the stench of rotting flesh it exudes in order to attract flies for pollination. It grows inside the roots or stem of its host and is visible only when it’s ready to reproduce and its flower erupts out of its host. STEM PARASITES There are two main types of stem parasitic plants: the mistletoe species and twining plants. Stem parasites are usually bigger than root parasites but less colourful.
Mistletoe attaches directly to its host tree while twining plants germinate in the ground before “climbing” the host tree to extract nutrients. Dodder (Cuscuta) is an example.
WHAT MAKES A GOOD HOST?
Different parasitic plants prefer different types of hosts. Some are specific and attach to a single host type, while others will attach to any available host and sometimes even to several different host types simultaneously.
Factors such as the host’s root system, the parasite’s germination and growth cycles, as well as the host’s defence mechanisms all play a part in the success of the parasitic plant.
Some plants have excellent defence mechanisms such as having almost impenetrable bark, secreting toxic chemicals that kill the parasite or chemicals that stop the parasite’s seeds from growing, which make it practically impossible for any parasitic plant to attach to it. But in most cases a plant’s defences aren’t adequate and it becomes an easy victim to a parasitic plant.
REPRODUCTION
Parasitic plants attract insects with their beautiful flowers and in many cases strange fragrances – such as the stench of rotting flesh.
Insects, larger animals and birds that hunt insects, as well as the wind help distribute the seeds and roots of these plants.
Some such as the dwarf mistletoe species (Arceuthobium) ensure their own distribution of seeds. Each fruit of the dwarf mistletoe has a single seed surrounded by a sticky liquid. As soon as the fruit ripens, pressure builds up until the thick skin bursts open and the seed is expelled at great speed – up to 90km an hour – for distances up to 15m.
The seed lands on its next host and germinates there. The sticky liquid helps it attach to the new host and the entire cycle starts again.