The loss of water vapour in transpiration, exchange of carbon-dioxide to oxygen in the leaf occurs through pores called stomata. Normally stomata are open in the day time and close during the night. The immediate cause of opening and closing of the stomata is a change in the turgidity of the guard cells. The inner wall of each guard cell towards the pore or stomatal aperture is thick and elastic. When turgidity increases within the two guard cells flanking each stomatal aperture or pore, the thin outer walls bulge out and force the inner walls into a crescent shape.

The diagrams show a transverse section of a stem, and a transverse section of a root. (c) Make a copy of the diagram that shows a transverse section of a root. Label the xylem tissue

Xylem vessels and phloem tubes are usually found close together. A group of xylem vessels and phloem tubes is called a vascular bundle. The positions of vascular bundles in roots and shoots are as follows: In a root, vascular tissue is found at the centre, whereas in a shoot vascular bundles are arranged in a ring near the outside edge.

The diagrams show a transverse section of a stem, and a transverse section of a root. (d) On your two diagrams, label the position of phloem tissue.

Xylem vessels and phloem tubes are usually found close together. A group of xylem vessels and phloem tubes is called a vascular bundle. Phloem tubes are made of many cells joined end to end. However, their end walls have not completely broken down. In a root, vascular tissue is found at the centre.