Root and leaf structure  (Page 4/5)

When a plant has sufficient amount of water and sunlight, guard cells accumulate potassium (K ⁺ ) ions, and water flows into the guard cells by osmosis. The movement of water into the guard cells causes the cells to swell and become turgid. As the cells become more turgid, the inner thick cell wall does not stretch as much as the thin outer cell wall, and this causes a space to form between the two guard cells opening the stoma. When the plant closes that stoma, K ⁺ ions leave the guard cells, and water follows by osmosis. The movement of water out of the guard cells causes the cells to shrink and become flaccid thereby closing the stoma. [link] is an illustration of how guard cells open and close the stoma.

The movement of K ⁺ ions into or out of the guard cell as currently hypothesized is a passive process and related to the active transport of H ⁺ ions in the opposite direction of the movement of the K ⁺ ions. So, when H ⁺ ions are actively pumped out of the guard cell, K ⁺ ions move into the guard cell passively flowing down an electrical gradient created by the H ⁺ ions. Since the plant uses energy to open and close the stomata, the benefits of regulating the opening and closing of the stomata are greater than the energy expenditure of moving ions into and out of the guard cells. Plants actively regulate the movement of these ions and can respond rapidly to changes in the amount of sunlight, relative humidity and carbon dioxide.

The epidermis is usually one cell layer thick; however, in plants that grow in very hot or very cold conditions, the epidermis may be several layers thick to protect against excessive water loss from transpiration. A waxy layer known as the cuticle covers the leaves of all plant species. The cuticle reduces the rate of water loss from the leaf surface. Other leaves may have small hairs (trichomes) on the leaf surface. Trichomes help to deter herbivory by restricting insect movements, or by storing toxic or bad-tasting compounds; they can also reduce the rate of transpiration by blocking air flow across the leaf surface ( [link] ).