Forestry : The Living Tree : Parts of a Tree : Leaves

[PARTS OF A TREE - LEAVES HIGHLIGHTED]

Parts of a Tree - Leaves

Leaves are the food factories of the tree. Powered by sunlight, the green substance in leaves called chlorophyll uses carbon dioxide and water to produce carbohydrates. This process is called photosynthesis . The word photosynthesis comes from two Greek words: 'photos' meaning light and 'synthesis' meaning putting together.

During photosynthesis, oxygen is released into the atmosphere through tiny pores called stomata. Water is also released from these pores in a process called transpiration. In one day, a large tree will release enough water vapour to produce 1300 liters of water. This is enough to provide more than four days worth of water for the average person. In this way, trees act like giant air conditioners, cooling the air with water vapour and expelling oxygen, which we need to breathe.

Did you know:
Forests play an important role in the water cycle. They pull water out of the soil and return it to the atmosphere as water vapour and they also store huge amounts of water, both in the trees and soil surrounding the roots. This helps to prevent flooding and in the spring, and reduces soil erosion.

Cross-section of a leaf:

A typical single leaf or leaf blade contains several different tissues. Cells that make up the tissue have acquired specialized functions (through differentiation). A waxy cuticle covers the upper and lower epidermis forming a protective coat which prevents water loss in the leaf. Palisade and spongy tissue constitute the mesophyll layer where photosynthesis occurs. The elongated cells of the palisade layer are located in the upper part of the leaf where the most sunlight would reach. These cells contain chloroplasts; organelles that contain chlorophyll. The spongy layer is a tissue of irregular, loosely arranged cells, containing fewer chloroplasts, with large air spaces between the cells. Stomates are located in the lower epidermis. Stomata allow for the movement of carbon dioxide, oxygen and water between the atmosphere and the plant. Two guard cells regulate the opening or closing of the stomates. It has been found that the concentration of carbon dioxide in the guard cells is associated with the opening and closing of these cells.

Respiration and photosynthesis play an important role in the CO₂supply. Respiration is the reverse process of photosynthesis. During respiration, sugar (C₆H₁₂O₆) is broken down in the cells and energy is released. This energy is used by the cells for several purposes. Respiration requires sugar and oxygen and one of the by-products released in carbon dioxide. When CO₂ accumulates during respiration, the pH within the guard cell falls and this favours the formation of starch from soluble sugar, reducing turgor and resulting in a narrowing of the pore. When light increases, photosynthesis uses up the CO₂, pH rises and starch undergoes hydrolysis. Water then enters the guard cell, increasing turgor and the pores widen.