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
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..
How many different organisms can you see living on the bark of these
Above and left: some of the fine green powder from the bark
of an oak tree as seen under the microscope.
Link: The structure of bark.
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
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 ivy.
To study more about mosses click here.