Structure of the Leaves
The shape and the structure of the leaves of conifers, called needles, are different. They are narrow, elongate and either rhomboid, semi-circular or elliptical in cross-section in order to limit transpiration as much as possible.
Between the xylem and the phloem, there is a thin layer, called the cambium, consisting of several layers of thin-walled cells which, during the growth period, rapidly divide to form new wood on the inside and new phloem on the outside.
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.
This autumnal coloration is characteristic of many species of trees, e.g. the leaves of poplar, birch and common ash turn yellow, the beech turns orange-brown, the red oak and wild service tree turn dull red and the staghorn sumach red or yellow-red. A corky layer forms between the leaf stalk and the twig, severing the connecting tissues.
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.
