The Structure of Plant Roots
Investigating the Internal Anatomy of Roots
The tissues of mature roots can be divided into three types: epidermis, cortex, and vascular cylinder.
Epidermis
The epidermis is a thin outer layer of cells that take in water and nutrients. Epidermal cells grow slender projections called root hairs. Root hairs are simple unbranched tubes with rounded ends produced as outgrowths of the epidermal cells in the zone of maturation. Although individual root hairs are very small, even microscopic, there are an enormous number of them on a typical plant. Grass plant roots may possess billions of them.
Root hairs increase the absorbing surface of a root system as much as 20 times, and because they extend so widely through the soil, they make available a supply of water and minerals that the plant could not possibly obtain otherwise.
This intimate contact of root hairs and small rootlets with soil particles has a practical disadvantage to the gardener and nurseryman, however. In the process of transplanting, these rootlets and root hairs are torn from the plant, greatly reducing the plant’s ability to absorb water. This difficulty can be minimized by moving a block of soil with the roots, and by watering and partially shading new transplants.
Cortex
Just inside the epidermis is a thick layer of large loosely packed parenchyma cells (cube-shaped cells with large central vacuoles and thin cell walls) known as the cortex. Much of the bulk of the root is cortex, and these thin-walled cells serve as the path across which water and minerals pass as they progress from the outside of the root toward the vascular cylinder in the center of the root. As roots mature, a considerable amount of starch (stored food) usually accumulates in the many intercellular spaces of the cortex.
At the inner boundary of the cortex is a single layer of cells known as the endodermis. The endodermis wraps the vascular cylinder and stretches up and down the entire length of the root. Water and nutrients entering the epidermis pass into the cortex cells and move easily through and around them. Root cortex cells also have active transport mechanisms in their membranes that keeps water and nutrients moving deeper toward the center of the root.
Each endodermis cell is surrounded on four sides by a waterproof strip called the Casparian strip. Visualize a brick with a thick, sticky rubber band wrapped around its middle. The strips around the individual cells touch each other and stick together like the mortar between the bricks in a wall. Now imagine many such bricks placed side by side to build a cylinder.
The Casparian strips are impervious to water. Thus water and nutrients cannot move around the endodermis cells, only through them. Endodermal cells use active transport to pump dissolved nutrients into the vascular cylinder. As these dissolved nutrients build up inside the vascular cylinder, water moves through the endodermal cells by osmosis to equalize the relative concentration of water molecules in the tissues.
This active transport of nutrients coupled with the osmosis of water and the sealing of the spaces between endodermis cells by the Casparian strip results in a one-way flow of water and nutrients into the vascular cylinder.
Vascular Cylinder
The vascular (fluid-conducting) cylinder (tubes) is divided from the cortex and completely surrounded by one to several layers of cells called the pericycle The pericycle plays several important roles. In most seed plants, lateral roots arise from the pericycle and in those whose roots undergo secondary growth, such as woody dicots, the pericycle gives rise to covering and vascular tissues.
The center of the vascular cylinder is occupied by tube-like cells of two types: xylem which conducts water and minerals up the root and into the stem and phloem which conducts food molecules down from the stem into the root.
As more nutrients are pumped into the vascular cylinder, more molecules of water follow. Because neither can move back into the cortex, pressure known as root pressure builds up inside the vascular cylinder.
This increased pressure forces water into the xylem. As root pressure in the vascular cylinder forces more water into the xylem, and as more and more water moves from the cortex into the vascular cylinder, the water in the xylem is forced upward into the stem.
“We must learn not to disassociate the airy flower from the earthy root, for the flower that is cut from its root fades, and its seeds are barren, whereas the root, secure in mother earth, can produce flower after flower bringing their fruit to maturity.”
(Kabbalah)
Roots are the Unseen Dimension of the Plant Realm
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