Phloem

Plants have tiny tubes inside them.

These tubes carry food to the plant. The food is made in the leaves.

It travels to the roots and the fruit. This food helps the plant grow.

This food can even move in many directions. It helps the plant stay healthy and strong.

Plants have living parts that move food. This tissue is called phloem.

It is found inside the bark of trees. Phloem carries sugar to the rest of the plant. This sugar is made in the leaves.

The way food moves is called translocation. The sugar travels in a liquid called sap. This sap can move in many directions. It can go up or down.

This helps food reach the roots and the fruit.

Phloem has many different parts. Sieve tube elements are the main tubes. They carry the sugar through the plant. These cells do not have a nucleus. They need help to live. Small companion cells help them. These companion cells do most of the work. Phloem also has cells for support. Some are long fibers. Others are hard sclereids. These parts keep the plant strong.

If the phloem is stripped away, the plant can die. This is because the roots will not get food.

Plants have a special way to feed themselves. They use a living tissue called phloem to move food around. This tissue is found inside the bark of trees.

The phloem carries a sugary liquid called sap to every part of the plant. This sugar is made in the leaves during a process called photosynthesis. Without phloem, the roots and growing fruits would not get the energy they need to live.

The way food moves through the plant is called translocation. It works by moving sugar from a source to a sink. A source is a part of the plant that makes food, like a leaf. A sink is a part that needs food, like a growing fruit or a root.

In the spring, roots act as sources to help new growth. Later, the leaves become the main sources. Unlike the xylem, which only moves water upward, phloem can move sap in many directions. This allows food to travel up to flowers or down to the roots.

Scientists have studied how these tiny tubes work for a long time. A man named Carl Nägeli introduced the name "phloem" in 1858. He chose the name from an Ancient Greek word that means "bark."

The phloem is made of many different types of cells. Sieve tube elements are the main cells that carry the sugar. These cells are very special because they lack a nucleus when they are mature. They rely on smaller companion cells to do their metabolic work. These companion cells stay connected to the tubes through tiny openings.

Different cells give the phloem its strength and shape. Sieve tube elements have pores at their ends called sieve areas.

Some phloem cells are long fibers that provide strength without losing flexibility. Other cells are called sclereids, which are hard and irregularly shaped. These sclereids can make fruit like pears feel gritty when you eat them. There are also different types of companion cells. Transfer cells have folded walls to help move solutes, while intermediary cells have many vacuoles.

Knowing how phloem works helps us understand how plants grow and survive. If the bark is stripped away in a ring, it is called girdling. This process can kill a tree because the nutrients cannot reach the roots.

However, farmers sometimes use girdling on purpose. They can use it to make fruits or vegetables grow much larger. In the past, people in Finland and Sweden even used pine phloem as food during famines. They dried it and turned it into a type of dark bread.

Phloem is a specialized living tissue found in vascular plants. Its primary role is to transport organic compounds produced during photosynthesis. These compounds are known as photosynthates, and the most common is the sugar sucrose. This movement of nutrients is called translocation. In many plants, the phloem is located as the innermost layer of the bark. This is why the name comes from the Ancient Greek word "phloiós," meaning bark.

Translocation works through a process of phloem loading and unloading. The movement is driven by positive hydrostatic pressures. This differs from the xylem, which uses negative pressures or tension. The sap is a water-based solution rich in sugars. It moves from a "source" to a "sink." A source is a part of the plant that produces food, such as a leaf. A sink is a part that consumes or stores food, such as a root, a bulb, or a developing fruit.

During the spring growth period, storage organs like roots often act as sources. These provide energy to the many growing areas of the plant, which act as sinks. Once the growth period ends and meristems become dormant, the leaves become the main sources. In this stage, the storage organs and developing seeds become the sinks. Because the flow is multidirectional, sap can move up or down. This means sap in adjacent sieve tubes might even flow in opposite directions.

Phloem tissue is composed of several distinct cell types working together. The main conducting cells are called sieve tube elements. To allow fluids to move easily, these cells lack a nucleus and most organelles at maturity. They do contain vacuoles and ribosomes before they mature, but these migrate to the cell wall and dissolve. The ends of these cells have pores called sieve areas. These pores are reinforced by platelets of a polysaccharide called callose.

Because sieve tube elements lack a nucleus, they depend on companion cells. These are specialized parenchyma cells that handle the metabolic needs of the sieve tubes. There are three types of companion cells. Ordinary companion cells have smooth walls. Transfer cells have highly folded walls to help scavenge solutes using energy. Intermediary cells possess many vacuoles and help synthesize certain oligosaccharides. In some plants, like gymnosperms, albuminous cells perform a similar role to companion cells.

Other cells in the phloem provide mechanical support through sclerenchyma. This includes long, narrow fibres and irregularly shaped sclereids. Fibres provide tensile strength and flexibility. Sclereids add compression strength and can protect against herbivores. For example, sclereids create the gritty texture found in pears. The term "phloem" was introduced by Carl Nägeli in 1858. He used it to distinguish the tissue formed outwardly by the cambium from the xylem.

Understanding phloem is vital for studying plant survival and agriculture. If the bark is removed in a ring, it is called girdling. This process kills the plant because nutrients cannot reach the roots. However, farmers use girdling to produce massive fruits. By removing all but one fruit on a branch, all the branch's sugar is directed to that single piece. Historically, people in Finland and Sweden used pine phloem during famines. They dried and milled it into a flour called pettu to make dark bread.