Avocado (Persea americana) is a tree and the fruit of that tree, classified in the flowering plant family, Lauraceae. It is native to Central America and Mexico. The tree grows to 20 m (65 ft), with alternately arranged, evergreen leaves, 12-25 cm long. The flowers are inconspicuous, greenish-yellow, 5-10 mm wide. The pear-shaped fruit is botanically a berry, from 7 to 20 cm long, and weighs between 100-1000 g. It has a large central seed, 3-5 cm in diameter. An average avocado tree produces about 120 avocados annually. The fruit is sometimes called an avocado pear or alligator pear, from its shape and green skin. The avocado tree does not tolerate freezing temperatures, and so can be grown only in subtropical and tropical climates. The avocado fruit does not ripen on the tree, but will fall off or be picked in a hard, green state, then it will ripen quickly on the ground, but depending on the amount of oil that it has the taste may be very different. Generally, the fruit is picked once it reaches a mature size, and will then ripen in a few days (faster if stored with other fruit such as bananas, because of the influence of ethylene gas). The fruit can be left on the tree until required, rather than picked and stored, but for commercial reasons it must be picked as soon as possible. If the fruit stays on the tree for too long it will fall to the ground.