The canopy is the topmost layer of the woodland and is comprised of the leaves of the dominant trees. In
these woods, these will be mostly oaks, but also beech, lime, birch, ash, maples and sycamores, yew trees
and pines and chestnut trees. Trees compete to reach the light and when grown in dense groups the trees
will grow thinner but taller in an effort to outreach their neighbours. Much of this light is absorbed by the
leaves and used to make sugars and other organic chemical building-blocks by photosynthesis.
Photosynthesis needs a source of carbon and leaves absorb carbon dioxide gas from the air. The leaves of
plants gives them an enormous surface area for light and carbon dioxide capture, whilst keeping weight to a
Left: the underside of an oak leaf.
Above: a close-up view showing
islands of cells (areoles) surrounded
by the smallest vessels (which are
blind-ending). The central midrib and
veins help support the leaf but also
conduct xylem sap to the
photosynthesising cells in xylem
vessels. This xylem sap contains water
and mineral nutrients absorbed by the
roots. The vessels also contain phloem
vessels, the phloem sap carries the
sugars made by the photosynthesising
cells to other parts of the plant.
Above: the leaf of small-leaved lime, Tilia. The smallest areoles are about 1 mm in
diameter, or the diameter of about 20 cells.
Above: areoles of a copper beech leaf. Although the leaves of copper beech
contain plenty of green chlorophyll, it's colour is masked by the high concentrations
of red carotenoids in these leafs. Why are plants green?
Above: a clear-varnish cast of the underside of a holly leaf seen
under the microscope. Clearly visible are the large number of pores
or stomata (singular stoma) that allow the leaf to obtain carbon
dioxide from the air for photosynthesis. Most leaves have more
stomata on their under-surface and holly has no stomata at all on the
upper surfaces of its leaves.
Above: a close-up view of the varnish cast of three stomata. Each
stoma is flanked by two sausage-shaped guard cells. These cells can
change shape (by swelling or shrinking) in order to regulate the
diameter of the central pore (stoma) by opening or closing it.
Above: a beech tree (Fagus sylvatica) canopy.
Above: the canopy of a mixed beech, ash and hornbeam wood.