Vascular transport in plants

Hungry? When you eat and drink, you take in water and all nutrients through your mouth... or you would be doing something wrong, because you're human. BUT If you were a tree... You would get water and minerals from your roots, but other food - sugar that is - from your leaves.

Water, minerals, and food are carried throughout the tree by a transport system, the vascular system. The first plants on land, Bryophytes - moss, liverworts, and hornworts - showed up about 450 million years ago, together with ferns. Both bryophytes and ferns reproduce using spores. They are spore plants. Even the earliest ferns however, had something that bryophytes lacked: vessels in their roots, stem, and leaves.

They are vascular plants. After ferns, in time, more vascular plants developed: conifers, and flowering plants - which also include plants like grass and deciduous trees. The thing that distinguishes a vascular plant is its transport system: the vascular system. But how does it work? Well, vascular plants have two kinds of vessels: Vessels that transport water and minerals from the roots.

We call these Xylem... ... and vessels that transport the sugar glucose from the leaves. We call these phloem. The Xylem and the Phloem run together in something called vascular bundles. Inside the vascular bundles they are separated by a tissue layer, the Cambium.

Xylem is made of hollow tubes from dead cells set in a dense continuous row. The dead cells are either tracheids or vessel cells, depending on the kind of plant. The Xylem cells' walls are hard and stiff. This provides support for the plant - almost like your skeleton; stops the cells from collapsing under the weight of water being transported; and makes the cells waterproof. The walls are this hard because they are strengthened by the substance Lignin.

The walls are lignified, or woody. But if the Xylem is made of dead cells, is that also true for the Phloem? No, the phloem is made of living cells: tiny tubes in a row. Every tube has small pores in each end, where it connects with the next tube, to let the fluid through. The plant makes it's own food by combining carbon dioxide from the air with water from the ground, using sunshine.

The carbon dioxide and the water are converted into the sugar glucose - the plant's food - and oxygen, which is released into the air. So it's glucose in fluid form, the plant's sap, that is transported in the phloem. Too hot? When you get too hot you sweat and release water. Vascular plants also release water.

It's called transpiration. It's through these tiny openings in the leaves, Stomata, that the plant absorbs carbon dioxide from the air, and releases oxygen created by photosynthesis. But it's also through the stomata that the plant releases water. These, which might look like lips around each opening, are guard cells. It's the guard cells that regulate the size of the stomata and thereby the amount of water that is able to evaporate.

If it's a cool day, the stomata are fully opened, but on a warm day they are shut, so that the plant can conserve water. When water is released, it creates pressure in the Xylem, and this is what draws water from the roots to replace the water that has been released. This is called transpiration pull. A tree's vessels are situated just inside the bark. One common method used to kill trees is to remove a ring of bark and vessels around the trunk, so the tree will die before it is cut down.

Don't worry Maria. We won't ring-bark you. So now you know how vascular plants eat, drink, and sweat. It's very clever, but I can understand if you prefer to do it in your way.