Roots and Stems
Woody stems and mature roots are sheathed in layers of dead cork cells impregnated with suberin - a waxy, waterproof (and airproof) substance. So cork is as impervious to oxygen and carbon dioxide as it is to water.
However, the cork of both mature roots and woody stems is perforated by non-suberized pores called lenticels.
These enable oxygen to reach the intercellular spaces of the interior tissues and carbon dioxide to be released to the atmosphere.
Lenticels are also present on many fruits, quite noticeably on many apples and pears. On European pears, they can serve as an indicator of when to pick the fruit, as light lenticels on immature fruit darken and become brown.
In many annual plants, the stems are green and almost as important for photosynthesis as the leaves. These stems use stomata rather than lenticels for gas exchange.
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