Plant roots start out with undifferentiated cells as the new plant is developing, and those cells grow and differentiate into more specialized cell types. The root, cut in a cross section, reveals several different specialized layers with different kinds of cells. Plants’ roots serve to anchor the plant in the soil, to absorb water and nutrients and to store food for the plant.
Plants’ roots have an outer layer of cells that botanists call the epidermis. Unlike animal cells, plant cells feature a cell wall. The cell wall is a more or less firm structure composed primarily of cellulose, a substance formed from many glucose sugar molecules all connected together. An important feature of the plant root’s epidermal cells is the so-called root hair. The root hair is a thin, tubular outgrowth of epidermal cells. It grows between soil particles and serves to draw in water and nutrients from the soil.
Cortex and Endodermis
Whereas the epidermis may be only one cell layer thick, the cortex is a layer inside the epidermis that is multiple cells in thickness. The most interior layer of cells in the cortex is specialized. This layer is termed the endodermis, and it functions as something of a filter. From the outer epidermis and through the cortex, water can make its way into the root by simply passing through the porous cell walls of adjacent cells. This is analogous to water being able to move through a brick wall by going freely through the mortar between the bricks. The bricks represent the cell, and the mortar represents the interconnected cell walls.
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The root’s endodermal cells have a strip inside their walls that is impervious to water. Botanists call it the Casparian strip. It forces water that is entering any deeper inside the root to go through the cell contents of endodermal cells rather than merely passing through the cell walls. The intricate interior of the cell can, in this way, exert some control or regulation over the passage of materials into the interior of the root.
Just inside the endodermis is a layer of cells botanists called the pericycle. This layer doesn't have a particularly specialized function unless the root develops later into a woody root. In that case the pericycle layer is involved in producing so-called secondary, or lateral, roots.
Xylem and Phloem
Inside the pericycle's ring of cells, in the innermost portion of the root, are the so-called xylem and phloem cells. These cells form what botanists call the vascular tissue of the plant. Just as vascular structures in humans and other animals conduct blood, this vascular tissue in plants conducts fluids of vital importance to the plant's survival. Individual xylem and phloem cells don't function independently, but serve as a collective to conduct water and food from cell to cell throughout the plant. The xylem cells conduct water and phloem cells conduct food, in the form of sugars, that the plant has manufactured by photosynthesis. Botanists may refer to these vascular cells in the root, collectively, as the “stele.”
Plant roots have a few other types of cells, or regions of cells, that are specialized. These include zones of growth and a collection of cells with thick cell walls forming a cap over the growing root tip. This root cap serves to protect the more delicate layers behind it as the tip grows and pushes into the soil.