A new layer of wood is added in each growing season, thickening the stem, existing branches and roots.A commonly applied narrower definition is that a tree has a woody trunk formed by secondary growth, meaning that the trunk thickens each year by growing outwards, in addition to the primary upwards growth from the growing tip.
In looser definitions, the taller palms, tree ferns, bananas and bamboos are also trees.
Trees have been in existence for 370 million years.
Under such a definition, herbaceous plants such as palms, bananas and papayas are not considered trees regardless of their height, growth form or stem girth.
Certain monocots may be considered trees under a slightly looser definition; Trees have modified structures such as thicker stems composed of specialised cells that add structural strength and durability, allowing them to grow taller than many other plants and to spread out their foliage.
Trees and forests provide a habitat for many species of animals and plants.
Tropical rainforests are among the most biodiverse habitats in the world.It is estimated that there are just over 3 trillion mature trees in the world.A tree typically has many secondary branches supported clear of the ground by the trunk.Trees play a significant role in reducing erosion and moderating the climate.They remove carbon dioxide from the atmosphere and store large quantities of carbon in their tissues.Trees, as relatively tall plants, need to draw water up the stem through the xylem from the roots by the suction produced as water evaporates from the leaves.