Transportation

As mentioned earlier, the transport in higher plants is with the help of the vascular system. Thus, these plants are also called the vascular plants.

Materials to be Transported

The materials to be transported across the plant body are water, minerals and food. Apart from these nutrients, substances like the hormones also have to be transported.

Vascular Tissue

The transport of materials takes place through specialised tissue called the vascular tissue.

The vascular tissue is of two types:

• Xylem
• Phloem

Xylem

It is the vascular tissue that transports water across the plant body. Xylem is made up of four different types of cells. They are tracheids, vessels, xylem fibres and xylem parenchyma. Of these only tracheids and vessels are involved in the transport of water and minerals.

Tracheids

Tracheids are elongated dead cells that have sloping end walls. The cavity is empty as the cells are dead. The walls are thickened with a material called lignin. These thickenings are in different patterns. The cells are arranged end to end.

Vessels are also dead cells that have variously patterned thickened walls. These thickenings are due to lignin. The vessels are arranged end to end. The end walls of the vessels are either partially or fully dissolved. This results in the formation of long tubes that carry water.

The xylem vessels and tracheids together form long tubes that have a narrow diameter. Thus they function as capillaries (narrow tubes) to transport water.

Phloem

It is the vascular tissue that transports organic substances like sucrose across the plant body. It is made up of four types of cells - sieve tubes, companion cells, phloem fibres and phloem parenchyma. Except for phloem fibres, all the other three types of cells are living. Sieve tubes and companion cells are mainly involved in the transport of the materials.

Sieve Tubes

They are tubes formed by cells that are joined end to end. The end walls of these cells have perforations. The mature sieve tube cells are enucleated. The cytoplasm of the sieve tube cells is continuous through the perforations of the end walls. This helps in the transport of materials.

Companion Cells

They are smaller cells associated with the sieve tubes. They have dense cytoplasm and elongated nucleus. It is in contact with the sieve tube cell through pores in the wall.