The plant cortex is a layer of ground tissue found in the roots and stems of vascular plants. Situated internally to the plant’s outer protective layer, this tissue is composed of relatively unspecialized cells. Its role involves metabolic functions like storage and transport, as well as providing structural maintenance.
Location and Structure of the Cortex
The cortex is located between the epidermis (the outermost cell layer) and the central vascular tissues of a stem or root. This arrangement forms a cylindrical layer around the central core, known as the stele or vascular cylinder. Its position allows it to mediate the movement of substances and provide mechanical stability.
The cortex is primarily composed of large, thin-walled parenchyma cells. These loosely packed cells have large central vacuoles and create intercellular spaces that facilitate gas and water movement. In young stems, the outer cortex often contains collenchyma cells, which have irregularly thickened walls that provide flexible support without restricting growth.
The cortex itself can be differentiated into three layers. The outermost layer, just beneath the epidermis, is the hypodermis, which may contain supportive collenchyma or photosynthetic cells. The main body is the general cortex, consisting of parenchyma cells. The innermost boundary is a specialized, compact layer of cells that tightly surrounds the vascular cylinder.
Primary Functions of the Plant Cortex
A primary function of the cortex is storage. Its parenchyma cells store food reserves like starch and proteins, as well as large amounts of water in their vacuoles. This water storage helps maintain the plant’s hydration and turgor pressure. Edible roots and tubers are often composed largely of cortical tissue filled with stored carbohydrates.
The cortex facilitates the transport of water and dissolved minerals. In roots, water absorbed from the soil must travel across the cortex to reach the xylem, the water-conducting tissue in the central vascular cylinder. The loosely arranged parenchyma cells and the spaces between them provide a pathway for this lateral movement.
Structural support is another function, particularly in young stems and other non-woody parts of the plant. This support is achieved through two mechanisms. First, turgor pressure from water in the parenchyma cells provides rigidity. Second, collenchyma cells with their thickened walls offer flexible strength, allowing stems to bend without breaking.
Cortex Variations in Different Plant Parts
The cortex’s structure and role are adapted for different plant organs, most notably roots and stems. In roots, the cortex is much larger and makes up a greater proportion of the organ’s volume compared to the stem. This large cortical region is dedicated to storing food reserves and water.
A defining feature of the root cortex is its highly specialized endodermis. The cells of the endodermis possess a waterproof band in their walls called the Casparian strip. This strip is made of a waxy substance and forces water and minerals to pass through the cell membranes of the endodermal cells, rather than moving between them. This mechanism allows the plant to selectively control which ions enter the vascular cylinder for transport to the rest of the plant.
In contrast, the cortex in young stems is often thinner and more focused on support. A key distinction is that cortical cells in green stems contain chloroplasts for photosynthesis. This specialized photosynthetic tissue, known as chlorenchyma, allows young stems to produce carbohydrates, supplementing the photosynthesis that occurs in the leaves.