oak tree

How many different organisms can you see living on the bark of these trees?

Oak bark

Lichen and moss on oak tree

The bark of oaks is an important habitat for other organisms. On the bark of the oak trees in this woodland you can see at least three different types of epiphyte (an epiphyte is a plant or fungus that lives on the surface of another plant) - dark green moss, bright light-green Chlorella (a single-celled alga) and grey-green encrusting lichen (Lecanora shown below:

Lichens are ancient organisms, they colonise surfaces which other organisms find harsh, such as stones, desert rocks, the inside of rocks in the dry Antarctic deserts, salty and battered rocky shores and less harsh surfaces, like the bark of trees. Lichens are remarkable in that they are composite organisms comprised of two species fused together, one species is the fungus partner, which forms the bulk of the visible body or thallus, and the other is a microscopic alga or photosynthetic bacterium which harnesses light to manufacture organic compounds by photosynthesis. These organic compounds are utilised b y the fungus to construct the thallus that helps protect the algae from harsh conditions, such as episodic desiccation, and elevates the algae above the boundary layer of stagnant air so that spores can more easily be dispersed. Lecanora is one of the most pollutant-resistant lichens (being resistant to acidic sulphur dioxide gas) and so is only abundant on trees near cities.

Lecanora is a flattened encrusting lichen, but other lichen species are foliiose (leaf-like) or arborescent (treelike in the way it branches) like those shown on the branch below:

These more plant-like lichens tend to be more common in damp woodland where the air is free from pollution and under such conditions they may carpet the bark of trees. The bright-green powdery substance visible on the bottom half of the above branch is a mass of single-celled algae probably Desmococcus (Pleurococcus) and/or Chlorella, which is shown under the microscope below:

Above and left: some of the fine green powder from the bark of an oak tree as seen under the microscope.

Chlorella is one of the simplest eukaryotic algae. It consists of single spherical cells that reproduce by cell division (mitosis) doubling in number with each division, and the daughter cells often remain coupled together in groups of 2, 4 or 8. Some of the cells in the photos above are in the act of dividing - a plate of cell wall material can be seen dividing some of the cells into two, across the cell equator, but the cell has yet to divide completely into two new spherical cells. Algae are photosynthetic and, along with the lichens, Chlorella utilises some of the dim light that escapes through the tree canopy to make sugars and other organic building blocks by photosynthesis. Chlorella cells are able to survive prolonged periods of dryness, in which the cells cease growth, and they can easily be blown or carried from tree to tree as a fine powder. When the cells above were placed under the microscope they were dry and shrivelled but within minutes of contact with water they inflated, rehydrated and sprang back into life! Each cell in the picture above is about 5-10 micrometres in diameter (5-10 millionths of a millimetre). Chlorella is cultured and sold as health food - it grows easily and is high in protein.

slime mould preparing to sporulate

What is that strange yellowish foam-like substance on the bark of this oak tree? It must be about 30 cm (12 inches) long and breaks up to the touch like shaving-cream!

Ivy is another epiphyte - not only does it creep along the ground as part of the field layer, but it has touch sensors that detect nearby objects as it slowly waves its fronds about and then it grabs hold of whatever it finds and climbs over it or up it. Learn more about the dance between the oak and the

Mosses are bryophytes: plants that do not produce flowers but instead exhibit alternation of generations; that is they alternate between two forms, the gametophyte and the sporophyte. The gametophyte produces gametes (egg cells and spermatids). Fertilized egg cells give rise to sporophytes which produce capsules of spores; each germinated spore developing into a new gametophyte.

Many mosses are epiphytes with some species preferring a particular species of tree or even a particular location on a tree, such as at the base of the trunk or on horizontal branches. Key variables effecting the distribution of these epiphytes include the smoothness or roughness of the bark, pH, moisture (some barks retain moisture better as do some locations on trees, e.g. nooks and crannies), ambient light levels (open wood versus dense shade) and atmospheric pollution.


Above and below: the liverwort Metzgeria furcata (gametophyte) growing on the bark of a Maple tree (Acer). Liverworts either consist of creeping stems bearing rows of veinless leaves (leafy liverworts) or flattened bodies divided into lobes or branches (thalloid liverwort). Metzgeria is a thalloid liverwort common on bark, rocks and walls.


This liverwort was growing with at least three species of moss, including the feather moss Hypnum cupressiform (above bottom) and Orthotrichum and microscopic algae (such as Trentepohlia).



The thallus (body) of Metzgeria is only one cell thick.



The thallus bears minute hairs (cilia) on the under-surface and along the margins.


The under-surface of ripe parts of the thallus contain hairy protuberances/covers called involucres, as shown below:


Above:under-surface of Metzgeria, showing hairy protuberances, which are in fact the inflated involucres or covers that protect the female reproductive organs.

Looking inside one of these inflated involucres revealed a group of rotifers, attached by their adhesive pedal glands to the liverwort. The liverwort Frullania, which is also found on tree bark, has special cup-like structures that store water and often also house rotifers. It has been suggested that the rotifers, which feed on micro-organisms and other tiny organic particles, supply nitrogen to the liverwort in their waste. Minerals are hard to come by when living on the bark of a tree trunk, so the rotifers may be valuable allies.


Archegonia, the female reproductive structures, will eventually develop within the involucre. If an egg cell, within its archegonium, is fertilized by a spermatozoid (produced inside the male organs or antheridia and also called an antherozoid) then the sporophyte will develop as a spore capsule borne on a stalk which will emerge from beneath the involucre.


A close-up view of the thallus tip. The thallus is only one cell thick (except for the midrib) and about 1 mm in diameter. It contains a midrib of elongated cells which may function in the transport of nutrients but probably also have a supportive function. The one above is beginning to fork into two.


To study more about bryophytes click here.


Section through Tilia bark

Link: The structure of bark.

Elder bark

Link: Algae on Elder bark