In botany, pith is a term used to refer to a spongy, usually soft, substance that is found in the middle of the stems and roots of many plants. Another name for this tissue is the medulla. The medulla tissue is made up of a particular type of plant cell, called parenchyma cells, which tend to be relatively large, with thin cell walls. The main functions of this tissue are nutrient storage, and the transport of nutrients through the stem, branches, leaves, and roots of the plant. A secondary definition of the term pith is in reference to the pale, spongy inner layer of rind, more properly called albedo, which is found in citrus fruits.
New pith growth is usually soft, spongy, and white or pale in color. As the tissue ages, it commonly darkens to a deeper brown. Tree pith is generally present in young growth; in the trunk and older branches, it is often mostly replaced by a woody substance called xylem. In some plants, the medulla tissue in the middle of the stem may dry out and disintegrate, resulting in a hollow stem.
Most, or all, vascular plants, also known as higher plants, have at least some medulla tissue. A key characteristic of these plants is their ability to conduct water and nutrients throughout the structure of the plant, a function in which pith plays an important part. This ability to move substances up and down the stem, and along the branches, has allowed vascular plants to evolve into species that include specimens of a very large size. Non-vascular plants, conversely, which includes plant families such as mosses, liverworts, and algae, possess neither medulla tissue, nor a transport mechanism, and this means that they can only grow to a very limited size.
Humans have found numerous practical uses for medulla tissue. In Asia, the medulla of a shrub called the rice paper plant is used to make edible rice paper. Wicks for candles have been made from the medulla tissue of certain species of rushes. The pith of a plant called the sola, which grows in swamps in India, has even been used to manufacture lightweight helmets designed to protect the wearer from the heat of the sun. These pith helmets were once popularly worn by Westerners in tropical climates, and are still frequently used in Vietnam.