Cortex

Plants are particularly thin-skinned. Did you know that a plant’s epidermis is only one cell thick? Just under that skimpy outer layer is a plant’s cortex. [This is one reason why using dish soap on plants is such a bad idea.]

Cross-section of non-woody eudicot root (Kate Russell)

 Cortex description

The cortex is made up of thin-walled cells called parenchyma. Some of those cells are purposefully torn or separated to create air spaces. This porous tissue is called the aerenchyma [a-REN-ky-ma], from the Greek word for ‘infusion’. This word makes sense when you learn that the phloem is not the only part of a plant that transports nutrients. The cortex does, too! 

The cortex is responsible for transporting nutrients and carbohydrates into the central core of a plant’s roots through diffusion. But there is even more to the cortex than just nutrient transportation.

Functions of the cortex

Depending on the plant, cortical cells may also store carbohydrates, essential oils, latex, resins, and tannins. in many cases, the cortex also contains chloroplasts that are able to perform photosynthesis, converting carbon dioxide and water into simple carbohydrates. Taking things one step further, the cortex can then convert those simple carbohydrates into the complex carbohydrates found in bulbs, tubers, and root vegetables, such as beets, carrots, and turnips. The cortex also manufactures the bark seen on the outside of woody plants and the underlying cork. 

Cortex and water flow

In herbaceous plants, the innermost layer of the cortex is called the endodermis and the outermost layer is called the exodermis. The endodermis and exodermis are unique in that all of the cell walls have a woody band, called the casparian strip, except those facing the center or the outside of the plant. These casparian cells help regulate the flow of water between the vascular bundles, found just inside the cortex, and the outer cells of the cortex and epidermis.

Flax stem cross-section: 1. Pith 2. Protoxylem; 3. Xylem; 4. Phloem 5. Bast fiber 6. Cortex 7. Epidermis

Pests and diseases of the cortex

Several bacterial diseases invade the root cortex through injury sites and natural openings. These diseases include bacterial wilt of beans (Curtobacterium), ring rot of potatoes (Clavibacter), cucurbit bacterial wilt (Erwinia), black rot of cucurbits (Xanthomonas), and Pierce’s disease of grapes (Xyella). The Pythium oomycete, which causes blackleg, also moves through the cortex. Dry, brown lesions seen in the main or taproot cortex can indicate Fusarium crown and root rot.

The next time you cut a plant stem or root, use a magnifying glass or hand lens to see what’s really going on in there. There are some amazing things going on in there!