Elder stem. Light micrograph of a cross-section through the periderm of an elder (Sambucus niger) stem. The periderm is composed of three types of cells. The layer of cells third from bottom are the phellogen (cork cambium). This is a layer of actively dividing cells (meristemetic) which produce regular tiers of cells (pink). These cells become impregnated with suberin and die, forming a layer of cork cells (phellem, red). Cork is an impermeable layer that protects the plant.
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