Pines

All of the pines are woody plants. The mugo pine (Pinus mugo), native to the Alps of Europe, is one of the smallest pines. At maturity, it is really more of a bush than a tree, and is often planted in gardens of Europe and North America. Many other pines which are native to North America are large trees which can grow 197-262 ft (60-80 m) or more in height.

The leaves of all pines are needle-like and arise from the stem in bundles, called fascicles. Each fascicle is often associated with a fascicle sheath, a special tissue at its base. Most species have two to five needles per fascicle, but some species have as few as one and others have as many as eight needles per fascicle. The needles of pines are arranged in a spiral about the stem. Each year, as the branch of a pine tree grows, it produces a whorl of new leaves, called a candle. The needles of pines last about two years and most species are evergreen, meaning they have some needles at all times. Since pines have needles throughout the year, they have the potential to photosynthesize whenever conditions are suitable.

The needles of pines, like those of other conifers, are well-adapted for growth in dry environments. In particular, the outer surface of pine needles has a thick waxy layer, called a cuticle, which reduces evaporative water loss. Like the leaves of all higher plants, pine needles have special microscopic pores on their surface, called stomata, which are important for exchange of water vapor, carbon dioxide, and oxygen. The stomata are usually arranged in rows on the underside of the needles, where they appear as white lines. At the microscopic level, the stomata are beneath the surface cells, so they are often called "sunken stomata." This stomatal adaptation reduces evaporative water loss.

The pines are vascular plants, in that their trunks and stems have specialized cells, xylem and phloem, for the transport of water and food. The xylem of pines consists mainly of tracheids, elongated cells with thick walls and tapered ends. The phloem of pines consists mainly of sieve cells, elongated cells with relatively unspecialized sieve areas at the ends. Sieve cells are characteristic of gymnosperms and free-sporing plants, whereas sieve tube elements are characteristic of the more evolutionarily advanced flowering plants.