Woody stems and roots are covered by bark, whose outer layer is made of cork. Cork cells are dead and their walls contain suberin, a waxy substance that is impermeable to water and gases. This protects the stem and reduces water loss, but it creates a problem: the living tissues underneath, such as the phloem and cambium, still respire and need oxygen.The solution is lenticels. A lenticel is a small area of the bark where the cork cells are loosely packed, with many air spaces between them. Gases can diffuse through these spaces:• oxygen diffuses in to the living cells beneath the bark
• carbon dioxide from respiration diffuses outLenticels are usually visible as raised, pale spots or short lines on the surface of twigs and stems. They are easy to see as horizontal streaks on birch and cherry bark, and as small raised dots on elder twigs and the skin of potato tubers.Unlike stomata, lenticels cannot open and close. They have no guard cells, so they are always open. However, they are small and few in number, so the water lost through them is small compared with the water lost through stomata.Lenticels are important in roots too. Roots take in oxygen from the air spaces in the soil. In waterlogged soil the air spaces fill with water, which holds little oxygen, so root cells cannot get enough oxygen for aerobic respiration. This is why most plants grow poorly in waterlogged soil.
• carbon dioxide from respiration diffuses outLenticels are usually visible as raised, pale spots or short lines on the surface of twigs and stems. They are easy to see as horizontal streaks on birch and cherry bark, and as small raised dots on elder twigs and the skin of potato tubers.Unlike stomata, lenticels cannot open and close. They have no guard cells, so they are always open. However, they are small and few in number, so the water lost through them is small compared with the water lost through stomata.Lenticels are important in roots too. Roots take in oxygen from the air spaces in the soil. In waterlogged soil the air spaces fill with water, which holds little oxygen, so root cells cannot get enough oxygen for aerobic respiration. This is why most plants grow poorly in waterlogged soil.