Sunday 21 July 2019

Mistletoe not closely related to other wild plants

Mistletoe is an ancient pagan fertility symbol that still finds a place in Christmas celebrations
Mistletoe is an ancient pagan fertility symbol that still finds a place in Christmas celebrations

Jim Hurley - Nature Trail

Mistletoe is native to Britain and while it has been recorded in Ireland at a handful of sites it is not believed to be native to our green and pleasant land. Plants found growing in Ireland are taken to be introductions from either Britain or mainland Europe.

There have been so many introductions of Mistletoe by people in different countries, and different parts of countries, that it difficult to know where exactly the plant's native range begins and ends.

Mistletoe is defined as a hemiparasite. In Britain, it grows mainly on the branches of Apple trees in old orchards. Since it is not rooted in the soil, it was believed in the past that the plant was parasitic on the tree, sending roots into the branch to draw nutrients from its reluctant host.

It is now known that Mistletoe, like most other green plants, can make its own food via photosynthesis in its yellowish-green leaves. So, it plays a dual role: its makes its own food and it is partly or semi-parasitic. Any organism that has that dual way of feeding is known as a hemiparasite.

The Mistletoe 'roots' that penetrate the host are not true roots as we know them but are structures called haustoria. These root-like structures specialise in extracting water and nutrients. Interestingly, haustoria evolved independently worldwide in several unrelated groups of plants and fungi as the need for them arose.

Mistletoe is also unusual in that it is not closely related to other wild plants in this part of the world. It is placed in a family on its own and, in northern Europe, it is the only member in its family.

It fruits in winter around Christmas time. The fruits are round, are up to 10mm in diameter and are white in colour. Wild birds disperse the seeds. Birds eat the fruits, notably the appropriately named Mistle Thrush. The seeds inside are surrounded by a sticky juice and to rid them of adhering seeds, birds wipe their bills on tree branches thereby attaching the seeds to the bark.

The sticky juice around the Mistletoe seeds dries in the air and hardens fastening the seeds securely to the bark of the host tree. The seeds germinate, and a new plant emerges. When it is very young, the new plant depends on its own resources but as it grows its haustoria penetrate its host and help themselves to the tree's reserves of water and nutrients.