April 2018

At a recent Gardening Club meeting we were introduced to many rare and unusual wildflower species, including a few parasitic ones.  One feature that immediately sets wholly parasitic plants apart from other plants is that they don’t contain chlorophyll, the substance that gives plants their green colour and is responsible for photosynthesis which allows them to convert the sun’s energy into growth.  (Note that there are many semi-parasitic plants like mistletoe and yellow rattle that do contain chlorophyll which means that they are less of a drain on their hosts.)

Toothwort (Lathraea squamaria) was the first plant mentioned.   This plant may be found in Alexandra Park where I have seen it growing in the undergrowth below some of the trees there.  Because it contains no chlorophyll, it doesn’t need a sunny spot to thrive, just as long as its host gets plenty of sunshine, though it does need to attract pollinating bumblebees which it does mainly by scent.  It invades the roots of trees and spends most of its life underground, only sending up the flower spikes in early summer.  It has an underground network of rhizomes which have pad-like tips (called haustoria) that attach to, and penetrate the bark of the host roots to get at the nutrients within.  The whole of the plant, including the underground parts are a pale creamy white.  It is not clear whether it gets its name from the resemblance of the flowers to a row of teeth or from the tooth-like scales on its roots.  Like all plants whose name ends with –wort, it was used medicinally for toothache, based on the doctrine of signatures.  It wasn’t commonly used, however, due to the plant’s relative scarcity.

Another parasitic plant mentioned was common dodder (Cuscuta epithymum), a member of the convolvulus family.  I know dodder from Dungeness Nature Reserve where it parasitizes wood sage.  It takes a different approach to toothwort in that its life cycle is completely above the ground.  When the seed germinates, it produces a root-like shoot that must find a host quickly before the energy contained within the seed is exhausted.  Once it has found a host, a sucker develops from which a thread-like stem grows which twines around the host stem (always anti-clockwise, or widdershins as we used to say before clocks were invented) and haustoria in the sucker and stem penetrate the host’s stem to extract nutrients. The root-like shoot then dies off.  As it nearly always kills or at least severely weakens its host, further stems reach out to neighbouring plants until a fine network of dark red stems cover a sizeable area.  It has no leaves because, like toothwort, it has no chlorophyll, taking all the energy required for growth from its host plant.  Small clusters of flowers form along the stems and produce seeds for the next generation.  The seeds can lie dormant for many years and it seems to favour disturbed ground, making it a common plant of managed heathland.  Dodder has also been used in traditional medicine as a purgative and for liver disorders.  Though native to Europe, it is now found on six continents; its spread is thought to be due to exported forage.  

The third parasite mentioned was a bit of a surprise as I was not aware of a parasitic orchid.  The bird’s nest orchid is another parasite that does not contain chlorophyll and this plant is a real specialist.  It parasitizes a fungus Rhizoctonia neottiae that is only associated with beech trees.  It is probable that the fungus has a symbiotic relationship with the beech trees, and it is also possible that the orchid provides some nutrition to the fungus via its roots which mesh with the fungal threads.  Apparently in Finland it is one of three different orchids without chlorophyll that are not related and are believed to have developed the same life cycle independently.  After flowering, the orchid and its rhizomes die off leaving only the tips of its roots which then grow into new orchids.