Like animals, in growth and development plants are subject to influences of climate and nourishment. In its proper latitude, and with an abundance of water and food in the soil, a tree adds to its annual growth and lives to a great age. But when the soil becomes exhausted of the necessary elements, or a more robust species crowds roots and leaves, then a tree begins to show signs of decay. It is difficult to establish rules regarding the proper age for cutting. For timber, most trees are considered fit at about one hundred years, although oak may furnish excellent timber at two hundred years. The purpose for which the wood is to be cut determines the proper age. Young trees show a closer grain and give a more elastic wood than old ones. Very old trees, although apparently sound, are found to be partially decayed in the middle of the trunk, so that the elasticity and hardness of the wood are replaced by a characteristic brittleness.