24 How to Build a Tree-High

How to Build a Tree FOCUS Question How does a tree get the matter it needs to grow?

Food for Trees Imagine looking up at the tallest tree in the world— a California redwood named Hyperion. Its massive trunk reaches for the sky. Its evergreen branches tower 116 meters (381 ft.) overhead. How did Hyperion get so enormous? In order to grow, trees need to produce food. They do this through the process of photosynthesis. During photosynthesis, plants harness energy from the Sun to convert water and carbon dioxide into glucose and oxygen. Glucose is a type of sugar that is food for trees. Trees utilize some of this sugar to grow, producing matter in the form of leaves, wood, bark, flowers, cones, or fruit. Trees also store some glucose to use later as energy.

Photosynthesis occurs inside small structures in plant cells called chloroplasts. A tree takes in sunlight through the chloroplasts in its leaves. Chlorophyll, a green pigment inside the chloroplasts, absorbs this light energy. Water and carbon dioxide move into the chloroplasts. The tree's roots absorb water from the soil. The water travels to the leaves. Small pores in the leaves called stomata open to let in air. Carbon dioxide from the air combines with the water in a series of chemical reactions that produce glucose and oxygen gas. The stomata release the oxygen into the air as waste. The tree uses matter and energy from the glucose to grow larger and carry out processes needed for survival.

Growing Downward Even the tallest tree in the world started as a tiny seed. Seeds contain glucose stored by the parent tree. When a seed sprouts, the roots grow downward. Root hairs on the outer layer of each root absorb water and nutrients from the soil. The water travels to the tree's first leaves, where it is used for photosynthesis, providing more glucose for the tiny sprout. The roots use glucose to continue growing in search of water. The tip of each root is covered by a root cap. Under each cap is a root apical meristem (AY-pih-kul MEH-rih-stem)— an area where new cells form rapidly from existing cells that divide, or split in two. This primary growth causes the roots to become longer.

Growing Upward A shoot, made up of a young stem and leaves, also emerges from the seed and pokes up through the soil in search of sunlight. The stem grows upward in the same way roots grow downward. Cells at the shoot apical meristem use matter and energy produced during photosynthesis to divide rapidly. This primary growth at the tip causes the stem to become longer. Small bumps called buds form on either side of the stem. As the stem grows taller, some of the buds elongate into branches, each with its own apical meristem. Additional leaves form on the stem and branches, producing even more glucose. Trees use some of their glucose to produce cones or fruit. These structures contain seeds that may become new trees someday.

Growing Outward A tree also grows outward, becoming wider. Its stem, or trunk, experiences secondary growth in a layer of cells called the cambium. The cells in the cambium divide to produce long, tube-shaped cells called xylem (ZY-lum) and phloem (FLOW-um). As more xylem and phloem are created, the tree grows wider. Xylem transports water from the roots to the leaves to be used for photosynthesis. Phloem moves food from the leaves to other parts of the tree, where it is used for primary and secondary growth. Wowser! The Tree of Tule in Oaxaca (wah-HAH-kuh), Mexico, has one of the widest trunks in the world, with a diameter of about 11.6 meters (38 ft.). That's about as long as a school bus!