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4 Cards in this Set
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Cambium
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In botany the cambium is a layer or layers of tissue, also known as lateral meristems, that are the source of cells for secondary growth. There are two types of cambium
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Sapwood
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sapwood: a part of the wood, as distinct from the heartwood
Sapwood is the younger, outermost wood; in the growing tree it is living wood, and its principal functions are to conduct water from the roots to the leaves and to store up and give back according to the season the reserves prepared in the leaves. However, by the time they become competent to conduct water, all xylem tracheids and vessels have lost their cytoplasm and the cells are therefore functionally dead. All wood in a tree is first formed as sapwood. The more leaves a tree bears and the more vigorous its growth, the larger the volume of sapwood required. Hence trees making rapid growth in the open have thicker sapwood for their size than trees of the same species growing in dense forests. Sometimes trees (of species that do form heartwood) grown in the open may become of considerable size, 30 cm or more in diameter, before any heartwood begins to form, for example, in second-growth hickory, or open-grown pines. |
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Heartwood
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Heartwood is wood that has become more resistant to decay as a result of deposition of chemical substances (a genetically programmed process). Once heartwood formation is complete, the heartwood is dead. It appears in a cross-section as a usually colored circle, usually following the growth rings in shape. Heartwood may be much darker than living wood. However, other processes, such as decay, can discolor wood, even in woody plants that do not form heartwood, with a similar color difference, leading to confusion. Some uncertainty still exists as to whether heartwood is truly dead, as it can still chemically react to decay organisms, but only once (Shigo 1986, 54).
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Pith
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Pith is a substance that is found in vascular plants. It consists of soft, spongy parenchyma cells, and is located in the center of the stem in eudicots (both herbaceous and woody) and in the center of the roots in monocts. It is encircled by a ring of xylem (woody tissue), and outside that, a ring of phloem (bark tissue). In some plants the pith is solid, but for most it is soft. A few plants, e.g. walnut, have distinctive chambered pith with numerous short cavities.
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