
ee growth habit is an evolutionary adaptation found in different groups of plants: by growing taller,trees are able to compete better for sunlight. Trees tend to be tall and long-lived,some reaching several thousand years old. Several trees are among the oldest organisms now living. 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. They differ from shrubs,which have a similar growth form,by usually growing larger and having a single main stem; but there is no consistent distinction between a tree and a shrub,made more confusing by the fact that trees may be reduced in size under harsher environmental conditions such as on mountains and subarctic areas. The tree form has evolved separately in unrelated classes of plants in response to similar environmental challenges,making it a classic example of parallel evolution. With an estimated species,the number of trees worldwide might total twenty-five per cent of all living plant species.The greatest number of these grow in tropical regions; many of these areas have not yet been fully surveyed by botanists,making tree diversity and ranges poorly known. Tall herbaceous monocotyledonous plants such as banana lack secondary growth,but are trees under the broadest definition.trients and other chemicals to be distributed around the plant, and without it trees epidermis of the stem, woody plants also have a cork cambium that develops amo
