Phloem

Most leaves have veins that are easy to see. The pattern these veins take can help in plant identification. These veins, like our own, can be found throughout plants, from root to tip. There are two types of veins: the xylem and the phloem. Together, the phloem and xylem are called the vascular bundle.

Function of the phloem

Plant veins carry water, sugar and other nutrients. These veins are called the phloem and the xylem. Water and dissolved minerals are brought up from the roots in the xylem. Sugars (sap), created during photosynthesis, travel down the phloem to the roots. In trees, the phloem is also the inner layer of bark. Water can also travel up the phloem, but it is primarily associated with bringing food down the plant. I remember which is which with this:

 Water gets higher in the xylem, and food flows down the phloem.

Phloem cells make up the long, flexible fibers from plants, such as flax and hemp, that we use for clothing.

Phloem structures

Unlike xylem cells, which die but remain functional when they reach maturity, phloem cells are alive. Phloem tissues are specialized into sieve elements, companion cells, sclerenchyma, and parenchyma.

Sieve elements allow nutrients to flow into other plant cells. If the plant is an angiosperm (flowering plant), the sieve is made up of plates with spaces in-between. Sieve elements in gymnosperm and ferns have holes that allow nutrients to spread to nearby cells. Companion cells provide life support to the sieve elements through mutual openings. The sclerenchyma provides structural support to the plant and the parenchyma acts as padding material between the other cells and helps in the movement of food between the sieve elements and the companion cells.

Drawing of phloem structure (Kate Russell)

Potential for harm

Without the free movement of water, sugar, and other nutrients, a plant will die. The phloem is generally positioned closer to the outside of a plant than the xylem. This is why stripping the bark from around a branch or tree trunk (girdling) causes death. Without an unbroken line of phloem, nutrients cannot reach the roots.

Fungal diseases, such as stem blight, can plug up the phloem with spores, with similar results.