Xylem (pronounced zye’-lum) is a pipeline that carries water and nutrients from the roots of a plant to the leaves. The term comes from the Greek, xylos or wood. It’s part of a plant’s vascular system that also includes phloem.
Unlike most plant cells, the cells that form the xylem have no end walls. They grow end to end and form a tube-like structure through which fluids flow. In mature plants, the xylem cells die and become hard. For instance in trees, xylem actually becomes wood.
Xylem sap moves from roots to the leaves. It actually flows through the xylem system.
In addition to carrying water and nutrients from the roots into the leaves and stems of a plant, it also repolaces water lost through transpiration and photosynthesis.
The flow system is caused by three things:
Root pressure can actually lift water a couple of feet. Root pressure occurs mostly at night when the moisture in the soil is high. Breaking off a stem near the ground will usually reveal sap collecting on the injured stem. This is an example of root pressure. Sap can sometimes flow for days. By measuring the flow of sap, scientists can measure root pressure.
To understand capillary action think of how a sponge or porous paper soaks up water. That’s basically how capillary action works.
Transpiration of water from leaves actually helps pull the xylem sap. As water evaporates through the stomates in the leaves, it creates a sort of vacuum that pulls fluids up through the xylem. Transpirational pull can raise water in a plant for hundreds of feet upwards.
In addition to transportation of material, xylem also helps provide turgidity to support the stem and the leafy tops of plants.
As the plant matures, another layer covers the primary xylem, which itself becomes part of the wood of the plant.
There is considerable study in the subject of xylogenesis, which is how the xylem forms in various plants, particularly in food crops.
The counterpart of xylem is phloem, which comes from the Greek, phloos. Once nutrients and water have been transported from the roots to the leaves, the leaves make food through photosynthesis. Then, once that is done, phloem carries the food, mostly in the form of sucrose, back to all the plant’s parts, either for immediate use or for storage.
While xylem brings water and nutrients to the leaves for transformation into food through photosynthesis, phloem brings food back to plant parts. Interestingly, the Greek root word for phloem, phloos, also means wood.