Unit 2 How trees live and grow

2.1 Active vocabulary

Text 1

How Trees Live

A tree has three main parts. The roots anchor it in the ground. They absorb water and dissolve minerals. The trunk and branches carry sap and hold the leaves in the sunlight. The leaves make food. A tree grows higher and wider as its twigs and branches lengthen at the tips. Meanwhile the branches, twigs, and trunk grow thicker. Conifers and most deciduous trees add thickness.

Every year the cambium adds a layer of new cells to the older wood. Each layer forms a ring. By counting the rings one can tell the age of the tree (figure 2.1). They are thick in years of good rainfall and thin in poor years.

There may, however, be false rings, caused by interruptions of the water supply in the growing season. Drought, frost, fire, or disease may cause false or partial rings. A dry year may also result in a missing ring. A true annual ring can be identified by its sharp outer edge; a false ring, by its fuzzy border.

Water and minerals travel up from the roots to the leaves in the new layers of wood inside the cambium. Hence this part of the trunk is called sapwood (or xylem). Other sap carries food down from the leaves through a layer called phloem inside the bark.

Palm trees have no cambium. The woody fibers in the pithy mass of the trunk carry sap up and down. The trunk grows only at the top from a terminal bud. As the tree grows, the older sapwood stiffens with a hard material (lignin) and loses connection with the leaves.

Then it just stores water. At last it becomes solid heartwood. Heartwood makes the best lumber. If it decays, a tree surgeon can replace it just as a dentist fills a decayed tooth. A tree’s roots grow at the tips like branches and twigs. Many trees send a main taproot straight down. It may grow to a great depth seeking water.

While the cambium makes the tree trunk and its branches grow in size, the leaves produce the food that builds the tissues of the tree. Using the energy from sunlight, the green coloring matter in the leaves (chlorophyll) takes carbon dioxide out of the air. It combines the carbon dioxide with water and dissolved minerals from the roots to form sugars and starches. One cannot see the food-making process at work, but one can feel a result of it in the woods on a hot summer day. In the shade, the air is cool and fresh. The leaves cut off the glare of the sun and reduce heat by breathing out tons of water vapor into the air. This water was soaked up by the roots and carried to the leaves through the sapwood. The water not used in making food is breathed out into the air through pores in the leaf. Moreover, leaves purify the air by taking out carbon dioxide and giving back oxygen.

Text 2