October 2011 - A year in the life of trees
[Left] A very thin section of European Larch (Larix decidua) photographed under my microscope. The circle is about 3mm in diameter. The wood has been stained red to show up the cell structure.
From left to right can be seen the larger cells of spring growth, giving way to the smaller and more densely packed cells of autumn and winter. This is what explains the rings seen in wood. Each ring represents one full year's growth.
Larch is a conifer, more specifically a 'gymnosperm' and has a somewhat simpler structure of wood than broadleaf trees and the cells are box-like in cross section. Very unusually it loses all its needles (which are really just small leaves) every winter, hence the scientific name Larix decidua which hints at its deciduous habit.
[Right] Another very thin section of wood, this time English Oak (Quercus robur) at the same scale as above.
Again from left to right can be seen one full year's growth. In spring, when the tree opens out its new leaves and needs copious amounts of water, large conducting vessels develop. For the rest of the summer the cells and vessels are smaller as the tree settles into its new size and puts on a new shell of wood. Being a broadleaf tree it loses all its leaves in winter i.e. it is deciduous. Winter is marked in the wood and in this thin section by a sharp but wavy line at the left of the circle. This is followed by the large conducting vessels of the next spring's growth. Again this accounts for the rings in the wood.