The wood of deciduous trees contains numerous tracheae with large lumens, present in the annual rings. They are there for the water and fluid transport which is increasing during the growing season, because leaves, having a larger surface area, evaporate considerably more water than for example the needles of conifers. In the microscopic image, the regular arrangement of the early wood with wide lumina and that of the late wood with small lumina is striking.
Cambium* is formed by the layer of cells in the stem that are capable of cell division. The ability to grow of the cambium is stopped in late summer. This creates growth rings as boundaries between the dark late wood of the previous year and the bright newly formed early wood. The early wood is growing faster than the late wood and it therefore forms a broader zone of relatively large cells. The late wood, however, forms a narrow zone of smaller, thick-walled fiber cells. These two differently colored and unequally wide zones, form the increase of wood in the stem, within a year; the so called annual ring. On the basis of the annual rings, the age of a tree can thus be read out.
The cross section in the first image shows wide annual rings created by normal growth. The cross section in the second image shows narrow annual rings caused by drought.