Today we’re starting on transport in plants. Plants don’t have hearts or blood, but they still need to transport substances around because they are too big to rely on diffusion alone. The tissues involved in transport are the xylem vessels and the phloem vessels. In this article we will focus on the xylem and phloem structure.
Where are the xylem and phloem found?
If you take transverse cross-sections of a stem, leaf or root, it is easy to see both the xylem and phloem vessels. In the stem they are grouped together in vascular bundles – the xylem are towards the inside and phloem are towards the outside. You may also see other tubes called sclerenchyma fibres nearby. These fibres made from dead cells are just there to provide some extra support to the plant (they have lignin in their cell walls) and they do not transport anything.
In roots, the xylem is in an ‘x’ shape through the middle with the phloem filling the gaps. In leaves, the xylem vessels are found in the upper part of the veins (not the same as animal veins!), and phloem vessels in the lower part.
It is important to note that would this would not look the same for all types of plant. This is what you would expect to see in a herbaceous dicotyledonous plant (remember we mentioned those here).
Xylem vessels transport water and mineral ions up the plant. They also provide support to the plant because they are very strong. Xylem vessels are not a living tissue – they are just long tubes of dead cells with no end walls, forming a hollow tube which water can travel up in one unbroken column. The cell walls of the dead cells are strong because they contain a substance called lignin. There are small areas without lignin called pits where water and mineral ions can enter and leave the xylem vessels.
The phloem vessels have a slightly more complex structure. They transport dissolved substances (also called solutes or assimilates) such as sugars. In contrast to the xylem, the phloem is a living tissue. Each cell which makes up a sieve tube is called a sieve tube element. There are end walls between the cells called sieve plates – these have loads of holes in to allow substances to pass through easily. Although sieve tube elements are living, they only have a small amount of cytoplasm so that there is room for solutes to pass through the middle. This means they have no nucleus and only a few organelles of their own. So to give them a bit of support carrying out cellular functions, each sieve tube element has a companion cell. Companion cells generously carry out functions not only for themselves but also for the sieve tube elements. An important function is to provide ATP for active transport – more on that in a future article.
Some key differences between xylem and phloem structure are:
- Xylem vessels are dead cells, phloem vessels are living cells (sieve tube elements) supported by companion cells.
- Xylem vessels are for both transport and support, phloem vessels are just for transport.
- Xylem vessels have no end walls, phloem vessels have end walls called sieve plates which allow substances through.
Next we will look at the function of both the xylem and phloem in a plant.