The Water-Conserving Cuticle, the Epidermis

The epidermis covers the primary plant body. It secretes a waxy cuticle (a translucent coating) that covers the external surface of the plant to restrict water loss and resist microbial attack. Waxes are embedded in a matrix of cutin. This allows light to pass through but restricts water loss. Beneath the matrix are cellulose threads and a layer of pectins. If water loss by transpiration exceeds water uptake by the plant, dehydration and death can result.

Stomata are openings across the cuticle and epidermis that allow gases in and out. Guard cells in the epidermis can changc shape to create stomatal openings, which permit water and gaseous exchange with the air. Transpiration is controlled by the opening and closing of the stomata. Guard cells contain chloroplasts. When the level of carbon dioxide available for photosynthesis drops, potassium ions are transported into guard cells. Water follows by osmosis, causing increased turgor pressure. The increased pressure causes shape change, opening the stoma.

In sunlight, potassium and water move into the guard cells causing them to swell and create an opening for carbon dioxide entry (a benefit) and water loss (a detriment). At night, potassium and water move out, and the guard cells collapse to close the gap and conserve water.

The periderm (composed of cork cells that are no longer alive and are impregnated with suberin) replaces the epidermis when roots and stems show secondary growth.

This animation (Audio - Important) describes the function of stomata.

REVIEW: In daytime, most plants lose _____ and take up _____.

REVIEW: At night, most plants conserve _____, and _____ accumulates.

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