Let us now take a look at the structure of the tree trunk. In the centre, there is a narrow column of pith and around it a continuous shaft of wood, called the xylem. This is made up of concentric circles known as the annual rings. Then comes the thin layer of phloem and, on the outside, the bark, which in older trees may be split and furrowed.

This is an adaptation to the environment in which they grow, for they are trees of the north and of the mountains where the climate is harsh and the summer short. To make the best use of this brief period, and not to lose time producing new leaves, they generally retain their foliage throughout the winter.

The primary vein divides the blade into two, generally equal, halves. In some woody plants, however, the halves are not identical, especially at the base, and these are termed asymmetric (elm, hackberry). In other species the leaf may have several veins branching out from the base (maples); such leaves are usually palmately lobed.

The strip of compact summer wood is easily distinguished from the strip of the following year’s spring wood so that on a stump we can clearly see the yearly growth, in the form of annual rings, and thus easily determine the age of the felled tree.

The pine leaves live for a shorter time than those of the spruce; and leaves of a spruce growing at low elevations live longer than those growing in harsh conditions.

By examining the rings in trees hundreds and thousands of years old scientists can determine long-lasting changes in the weather and pinpoint alternating periods of dry and wet years in times about which we have no meteorological data. In trees growing in tropical regions where growth is continuous throughout the year the annual rings are hot usually so clearly discernible.