The leaves of woody plants consist of the stalk or petiole and the lamina or blade. The petiole facilitates movement and positioning of the leaf. The blade is the part where photosynthesis, respiration and transpiration take place. Though all have the same function, the leaves of the various species differ greatly in shape and size and serve as an important means of identification.
The leaves of broad-leaved trees may be cither simple, with a single blade (even if deeply lobed), or compound, with several small blades or leaflets from the same stalk.
Simple leaves may differ in outline and may be linear — a narrow leaf with parallel sides, at least 12 times as long as it is wide. Acicular — anarrow cylindrical leaf with a pointed tip, needle-like in form. Lanceolate — basically, a leaf shaped like the head of a lance, with the broadest part below the middle and tapered to a-point, but never less than 3 times as long as it is wide. Ovate — similar to lanceolate, but always wider, and never more than twice as long as it is wide; egg-shaped. Obovate — the reverse of ovate, the stalk rising from the narrow end. Orbicular — a rounded leaf as long as it is wide. Cordate — heart-shaped, referring to the lobed base of leaves. Rhomboid — diamond-shaped, or roughly so.
Compound leaves are described as trifoliate if they consist of three leaflets radiating from a single point. If there are five or more of these leaflets and if they radiate from the end of the petiole, the leaf is termed palmately compound. When the leaflets are arranged laterally in pairs on the main leaf stalk, the leaf is termed pinnately compound. There may be an odd number of leaflets with one located at the tip (terminal leaflet), e.g. the mountain ash, common ash, false acacia (black locust); or an even number of leaflet pairs, e.g. the honey locust and the like. In some species with large leaves the leaflets are divided even further and these are called bipinnately compound leaves, e.g. the honey locust or Kentucky coffee-tree.
The leaf base may be cuneate (triangular) (durmast oak), rounded (common pear), cordate (heart-shaped) (lime tree) or auriculate (eared) (English oak) and the apex acute (birch), acuminate (slender-pointed) (black poplar), rounded (aspen) or truncate (blunt-ended) (black alder). The leaf margin may be either entire (false acacia, magnolia), serrate (cherry, bird cherry), doubly serrate (hornbeam, grey alder), dentate (crack willow), or lobed (oaks).
A further distinguishing feature of the leaf blade is the pattern of veins, the system of vascular bundles that supplies the leaf with water and food material. In most woody plants the leaf venation is net led. With a single primary vein and several secondary veins branching off at intervals. 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.
Most European broad-leaved trees are deciduous, in other words, they shed their leaves in the autumn. Only in southern, and in temperate parts of western Europe do some trees retain their leaves throughout the winter, e.g. the common holly, the laurel and the box. In the autumn the organic substances produced by the leaves are concentrated in the body of the tree and the leaves begin to change colour as a result of the decomposition of the chlorophyll and growing predominance of the red and yellow carotenoid pigments, along with the increase of anthocyanin in the cellular sap. 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 leafstalk and the twig, severing the connecting tissues. The leaf then falls to the ground, giving back to the soil a substantial part of the minerals taken from it. The shedding of leaves is the result of the climate in these latitudes, where in winter trees other than evergreens limit their life processes to the minimum, eliminating the water in their tissues in order to withstand better the harsh weather of the cold months.
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. 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. To be able to bear the weight of the snow and survive frost and lack of water, the leaves have a different shape and structure. Most of their cells are thick-walled and their surface is often protected by a waxy layer. They are able to close their respiration pores (stomata) so perfectly that in winter the conifer passes off less water than a leafless broad-leaved tree. However, even conifers do not retain the same leaves throughout their entire lifetime. Depending on the species, leaves may live from 2 to 10 years, the oldest being shed annually. 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. The process is slow and gradual, and the tree appears unchanged. Only the amount of needles on the ground beneath it serves as an indication of its shedding rate.