secondarygrowth213-page11

secondarygrowth213-page11 - all starting with the letter...

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Secondary Growth in Stems: Wood, Bark and Surface Features - 11 Bark Although the bulk of secondary growth occurs in the xylem (wood), there is a second major area of secondary growth, the bark. The bark comprises all regions of the secondary growth stem exterior to the cambium. This includes phloem from the vascular cambium, and periderm (cork) tissues. Cork (Periderm) Primary growth produces the epidermis for protection and collenchyma cells for strength. However, as the stem enlarges with secondary growth, the primary tissues cannot grow, and must be replaced. Cork cambium, which originates from parenchyma-like cells of the outer cortex (or rarely epidermis cells), produces the cork tissue. Cork is also called phellem and the cork cambium is also called phellogen. The cork, cork cambium and cork parenchyma are collectively called the periderm. For reasons unknown, someone fond of terms provided duplicate names for all components of the secondary dermal tissues,
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Unformatted text preview: all starting with the letter “P”. Characteristics of cork • Several layers of thin walled and flattened cells that die at maturity • Walls have suberin (waxy fat impermeable to water; the same chemical found in the casparian strips of the root endodermis layer, and in the abscission zone in leaves. Walls may also contain lignin.) • Provides mechanical protection to stem • There may be a region of parenchyma cells produced interior to the cork cambium, too. The cork parenchyma cells can be called phelloderm (another term that can be ignored). Early cork Lenticels Lenticels in Bark Cork is generally impervious to fluids and gases so that special structures for gas exchange are required to provide oxygen to the living cells of the secondary growth region. Lenticels are weak "eruptions" of parenchyma cells through which gases can diffuse. Lenticels also contribute to the appearance of bark...
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