When speaking of the king of trees in Western world, oak tree undoubtedly takes the crown. The tree's main trunk grows upward and branches out toward the top. Under optimal conditions oaks can grow to a height of more than 130 feet（40 m) and live an average of 600 years. How do these big trees absorb nutrition for it's energy source? Firstly,  the leaves absorb carbon dioxide and produce sugars by means of photosynthesis. At the same time, transpiration in the leaves pulls the xylem sap upward. Oak trees also possess deep, broad root system which enable them to absorb water and minerals in the soil. Self pollination is another character of oak trees--they produce both male and female flowers. While male flowers are hanging notably on the trees, female flowers are hidden among the leaves