Transport in plants

“Always remember, someone’s effort is a reflection of their interest in you.”

WELCOME TO another lesson. I hope the first three introductory lessons whetted your appetites and you have been anxiously anticipating this one (I can just see the eyes rolling to the top of the heads).

Last term we concluded our study of the transport system in humans. Recapping for those who are just beginning the journey, the different elements of the transport system are: the heart, which functions as the pump; the blood vessels, arteries and veins are the tubes leading from and to the pump; and the fluid that is carried by the tubes is the blood. The blood is the only liquid tissue in the body and comprises red blood cells, white blood cells and platelets floating in the blood plasma. You will need to practise drawing the longitudinal section of the heart as well as the different blood vessels. Learn the labels and the functions of the different parts so that you can make your diagrams annotated. Remember that you do not have to wait on your teacher to do this (you can beg any butcher for a heart). Practise makes perfect, so go ahead and get lots of practice!

This week, we will look at the structure of the transport systems in plants. The transport of substances takes place in two sets of tubes: the xylem and the phloem. These are found together in specialised structures known as vascular bundles. In the vascular bundle, they are separated from each other by another type of tissue, the cambium. The following diagram shows how the vascular bundles are distributed throughout the plant organs. Notice that vascular bundles are present in the roots, the stem and the leaves, but how they are arranged together to form the vascular bundle is different in these three areas of the plant. Pay careful attention to this, because you might be asked to identify these parts of the plant in an examination.

XYLEM

Xylem is found on the inside of the vascular bundle in the stem.

CHARACTERISTICS OF XYLEM CELLS

These are specialised cells which are very long. They have become so specialised that they are no longer referred to as cells, but as vessel elements. Additionally, they are:

■ Very narrow, their diameter is much less than their length.

■ The cells do not have cytoplasm, so they are hollow.

■ They do not have end walls, so the cells join end to end to form continuous tubes.

■ They do not have nuclei, so they are no longer living; they are dead.

■ Their cell walls are lignified.

FUNCTIONS OF THE XYLEM

■ Transports water and mineral salts up from the roots to all parts of the plant (transport is in one direction only).

■ The lignin in the cell walls provides strength for herbaceous plants, allowing them to be erect.

■ The lignified cells form the wood in trees.

PHLOEM

Phloem is found on the outside of the:

■ Vascular bundle and consists of two types of cells: phloem sieve tubes and companion cells

CHARACTERISTICS OF PHLOEM SIEVE TUBES

■ Sieve tubes are also long, but not as long as xylem tubes.

■ They have some cytoplasm attached to the walls of each cell.

■ They also do not have nuclei.

■ Sieve tubes, unlike xylem cells, do not have lignin in their cell walls.

■ They have end walls that are perforated, meaning they have holes in them, causing them to look like sieves, hence the name sieve tubes.

■ Sieve tubes always have smaller cells, known as companion cells, attached to them.

■ The companion cells provide all the requirements that the sieve tubes need; that is why they can remain alive.

FUNCTIONS OF THE PHLOEM

■ Transports sucrose and other organic compounds from the leaves, where they are made, to all the other areas of the plant (growing points, roots and stem). This means that in the phloem, transport is in all directions.

■ Other substances such as pesticides are transported by the phloem.

Have fun learning! Take good care of yourself until next week!