Often a secondary covering called the periderm forms on small woody stems and many non-woody plants, which is composed of cork (phellem), the cork cambium (phellogen), and the phelloderm. The periderm forms from the phellogen which serves as a lateral meristem. The periderm replaces the epidermis, and acts as a protective covering like the epidermis. Mature phellem cells have suberin in their walls to protect the stem from desiccation and pathogen attack.
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This is an imperfect fungus and its teleomorph has not been described. Disease symptoms appear on tubers, but not on the haulm (vine) or roots, and are limited to the periderm, composed of phellem, phelloderm and cortical layers that replace the epidermis of the tuber.Fahn A, ed.
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The phelloderm, which is not always present in all barks, is a layer of cells formed by and interior to the cork cambium. Together, the phellem (cork), phellogen (cork cambium) and phelloderm constitute the periderm.Dickison, WC. 2000. Integrative Plant Anatomy, Academic Press, San Diego, 186–195.
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This allows the plant to select the solutes that pass further into the plant. It thus forms an important barrier to harmful solutes. For example, mangroves use suberin to minimize salt intake from their littoral habitat. Suberin is found in the phellem layer of the periderm (or cork).
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Older phellem cells are dead, as is the case with woody stems. The skin on the potato tuber (which is an underground stem) constitutes the cork of the periderm. In woody plants the epidermis of newly grown stems is replaced by the periderm later in the year. As the stems grow a layer of cells form under the epidermis, called the cork cambium, these cells produce cork cells that turn into cork.
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What is commonly called bark includes a number of different tissues. Cork is an external, secondary tissue that is impermeable to water and gases, and is also called the phellem. The cork is produced by the cork cambium which is a layer of meristematically active cells which serve as a lateral meristem for the periderm. The cork cambium, which is also called the phellogen, is normally only one cell layer thick and it divides periclinally to the outside producing cork.
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Untreated cork panel Cork is an impermeable buoyant material, the phellem layer of bark tissue that is harvested for commercial use primarily from Quercus suber (the cork oak), which is endemic to southwest Europe and northwest Africa. Cork is composed of suberin, a hydrophobic substance. Because of its impermeable, buoyant, elastic, and fire retardant properties, it is used in a variety of products, the most common of which is wine stoppers. The montado landscape of Portugal produces approximately half of cork harvested annually worldwide, with Corticeira Amorim being the leading company in the industry.
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The cortex is the primary tissue of stems and roots. In stems the cortex is between the epidermis layer and the phloem, in roots the inner layer is not phloem but the pericycle. Tree cross section diagram From the outside to the inside of a mature woody stem, the layers include: # Bark ## Periderm ### Cork (phellem or suber), includes the rhytidome ### Cork cambium (phellogen) ### Phelloderm ## Cortex ## Phloem # Vascular cambium # Wood (xylem) ## Sapwood (alburnum) ## Heartwood (duramen) # Pith (medulla) Tecpán, Guatemala. In young stems, which lack what is commonly called bark, the tissues are, from the outside to the inside: # Epidermis, which may be replaced by periderm # Cortex # Primary and secondary phloem # Vascular cambium # Secondary and primary xylem.
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