National Geographic : 1955 Nov
Crown. Trees increase in height and spread by adding a new growth of twigs. Roots. Taproot an chors the tree. Lat eral roots absorb water, minerals, and nitrogen for cells that make food. Cambium. Slippery, sticky film between bark and wood forms yearly rings of new wood inward and new bark outward. These rings reveal the tree's age. Heartwood (duramen) was sap wood, now inactive. Superior hardness and density make it preferable to sapwood for furni ture and flooring. Trunk supports the tree. In largest specimens it may be 30 feet in diameter. Outer bark forms protective armor. The growth andwork ofa 1 R E Leaves produce sugar by photosynthesis, oxygen being a by-product. Millions of microscopic chemical laboratories (chloroplasts) trap radiant energy from the sun, carbon dioxide from the air, and water from the soil. A tree's oxygen renews the air we breathe. Sugar is used as a direct source of energy for the tree or transformed into starch, proteins, and other products, including wood, resin, gums, oils, waxes, and alkaloids. © National Geographic Society 67 Drawn by Gilbert H. Emerson 673 and you'll find the moist, sticky cambium film. Its cells are constantly splitting, making new sapwood on the inside, bark on the outside. The inner bark continuously converts to outer bark, assuming patterns which identify the different trees: rough as an oak, smooth as a beech, or shaggy as a cedar. Roots Drink Tons of Water The tree stands on its roots and through them draws in the tons of water required to satisfy the urgent thirst of its growing months. Conifers, elms, and birches generally reveal a flat and shallow root structure; most of the hardwoods, such as walnut, hickory, oak, and beech, thrust taproots straight down. Roots divide again and again, the fine rootlets splitting into a maze of hairlike termi nal strands that push out in the ceaseless search for water-and for oxygen, without which they perish. Roots grow best where they have to work to find enough to drink. The cumulative pressure of growing roots can split granite or move boulders. And the tree takes more than water from the soil; for its roots absorb nutritious minerals in solu tion and may get nitrogen from bacteria. The tree's waterworks handle a remarkable volume. A scientist once calculated that to build one ton of wood a tree has to soak up approximately 1,000 tons of water. Moisture pours into the atmosphere from a tree in an invisible fountain of vapor. Al most every tree throws off large quantities of surplus water. Cohesion (or water tension), osmosis, and the transpiration pull of leaves are the prin cipal forces by which a giant tree pumps up hundreds of gallons of water a day from the ground. A mature date palm in a desert may seem exempt from the tree's common thirst for water. Yet it may need as much as 200 gallons a day in the hottest months to main tain maximum vigor and yield. A tree is indeed an amazing mechanism. And, despite the dendrologist's knowledge, it remains a mysterious thing-as mysterious as any living work of nature.