Parasitic Plants

PimlicoDan
Published on March 23rd 2019
6
For some plants, generating their own food is just too much like hard work. At some point in their history, they forsook their own efforts and began to take advantage of those around them. They became parasites.
A small number of British wildflowers, as well as many tropical species, are parasitic, accounting for about 1% of all flowering plants.
Mistletoe: its white, sticky berries are wiped on branches by songbirds after feeding and germinate in-situ, high off the ground.

Veggie Vampires

Parasitic plants produce specialised roots, called haustoria, which connect them to the host plant and draw water and nutrients from its conductive tissue.

Parasites are either:

  • Obligate: a host is necessary for the plant to live.
  • Facultative: the plant can exist independently, without a host.

They are located on either:

  • The stem of the host.
  • The root of the host. These are far more common.

And are either a:

  • Hemiparasite: a plant which obtains some nutrients and water from the host but still photosynthesises.
  • Holoparasite: a plant that takes all it needs from a host plant and cannot exist without it.
Stem parasite haustoria taking nutrients and water from a host plant. This happens below ground in root parasites.
As you can imagine, it’s a cut-throat world out there. Although some parasites have evolved self-incompatibility, other species see no issue in parasitising each other. Parasitic plants have also been known to steal genes and even mitochondria from their host – there’s no honour among thieves!

Evolution

How certain plants evolved to become parasitic is up for debate. However, many facultative root hemiparasites (parasites that could complete their life-cycle without a host) grow in overcrowded areas like meadows or marshland, where staking a claim to live can be difficult.
By attaching itself to a host, a recently-germinated seedling can tap directly into a ready-made source of water and nutrients to get the best start in life.
These particular types of parasite, as well as mistletoe – an obligate hemiparasite – bathed in full sun and also have a higher rate of transpiration than their host species.
This meadow may look like a blissful halcyon, but competition has forced some plants into parasitism. Plants which produce their own food are called autotrophs.

Bringing Up Baby

Interestingly, tests on certain parasitic plant seedlings found that they would forage using chemical cues to find the most nutritious host.
As the seeds of many parasitic species are tiny and dust-like, with negligible food reserves, connecting the correct host is vital.
Mistletoe famously uses birds to distribute its seeds, which they either wipe from their beaks onto the branch or pass through their bodies.
Mistletoe seedlings take a long time to germinate and develop, taking around five years to produce their first fruit.

British Parasitic Plants

Yellow rattle (Rhinanthus minor): Cheery yellow flowers and charming rattling seedpods – a signature sound of summer meadows – this species is a root hemiparasite, though it can exist without a host, too.
Mistletoe (Viscum album): Immediately recognisable, mistletoe is an obligate stem hemiparasite, taking water and nutrients from a host tree while photosynthesising through leaves of its own. It prefers apples, hawthorn or poplar as hosts and can become a pest in orchards.
Common cow-wheat (Melampyrum pratense): Delicate in appearance, with cute yellow flowers, cow-wheat has a mutually beneficial relationship with wood ants. It produces a sweet liquid below its flowers for them to eat. In turn, they defend the plant from being eaten and distribute their seeds.
Dodder (Cuscuta epithymum): A stem holoparasite that spreads like red spaghetti over the host plant, usually heather or gorse. Pretty pinkish-white flowers festoon stems in summer. Also known as devil’s guts.
Dodder grows as an annual and is a member of the bindweed family
Marsh lousewort (Pedicularis palustris): An annual or biennial hemiparasite. It feeds on a variety of hosts and is usually found in boggy ground. They have reddish-green pinnate leaves and purple flowers, similar in shape to the yellow rattle (hence also being known as the red rattle).
Broomrapes (Orobanche spp.): Talk about a dysfunctional family. There are many types of broomrape, all named after different host plants. Unusual flowers sprout from the earth in a range of colours and all are obligate root holoparasites.
Toothwort (Lathraea squamaria): Sometimes confused with broomrape, the sallow pinkish-white flowers appear around the base of hazel or alder hosts. An ugly beauty; the flowers are indeed somewhat toothlike.
Ivy broomrape, O. hederae. Although not a common find, it is more frequently found in southern England and Wales.

A Fungal Feast

Some parasitic plants don’t feed on a plant, but rather a fungus. The term for this is myco-heterotrophy, although you may occasionally still hear the name ‘saprophytes’ incorrectly given to plants which grow in this manner.
Often the host fungus is also a parasite. This means that through the host fungus, the myco-heterotroph is indirectly parasitising a plant, a term called epiparasitism.
It’s complicated stuff and, although seen very rarely, we have a few epiparasitic species in the UK. These include two orchid species: the birds-nest orchid (Neottia nidus-avis) and the ineffably rare ghost orchid, (Epigobium aphyllum) as well as yellow bird’s nest (Monotropa hypopitys), a very distant relative of heather.
The ghost orchid was officially declared extinct in the U.K. in 2005 but rediscovered in 2009 after a 23-year gap in sightings.

Exotic Eaters

Corpse flower (Rafflesia arnoldii): This parasite holds the record for the largest single flower in the world and can reach a metre in diameter. It has no leaves, no stems and no roots. Instead, it lives in the root veins of its vine host.
The only sign of its existence is the colossal red flower which fills its Asian jungle home with a putrid pong to attract pollinators. Despite this stench, it is one of Indonesia's national flowers (pictured, main image).
Australian Christmas tree (Nuytsia floribunda): A relative of our native mistletoe. This plant is a giant parasite, growing to almost tree-like proportions as its roots feast on a variety of neighbouring plants.
It's a sneaky devil, but its golden blooms, which cover the plant at Christmas time, are spectacular (see article below).
Witchweed (Striga spp.): A rather unassuming plant compared to those above, this facultative root hemiparasite is becoming a big issue in agriculture.
Witchweed’s host plants include plants we eat – sorghum, rice, maize and sugar cane. It weakens plants before they can produce a crop.
Jakkalskos (Hydnora africana) Possibly the ugliest flower in the world. Gruesome red flowers rupture from the barren earth of south-west Africa and emit a fetid stench to attract dung beetles. Fascinatingly unpleasant!
Hydnora looks as though it has come directly from the bowels of hell - and smells like it, too.
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