Tree sapwood cells

Just under the bark of a tree is a thin layer of cells, not visible to the naked eye, called the cambium. Here, cells divide and eventually differentiate to form bark tissue outside of the cambium and wood or xylem tissue iaside of the cambium. This newly formed wood on the iaside contains many living cells and conducts sap upward ia the tree, and hence, is called sapwood. Eventually, the inner sapwood cells become iaactive and are transformed iato heartwood. This transformation is often accompanied by the formation of extractives that darken the wood, make it less porous, and sometimes provide more resistance to decay. 

Water in the Living Tree. Wood in the living tree is formed and functions in an essentially water-saturated environment. The functioning sapwood cells are a part of the vascular system that conducts water and solutes from the roots to the leaves through a continuous water-saturated network of wood cells (2). When the tree is felled the water in the wood is cut off from the soil water and the wood commences to lose most of its moisture. 

The central wood portion of the log depicted in Figure 1 is considerably darker in color than the part adjacent to the bark. The dark-colored wood is termed heartwood and the light-colored wood is termed sapwood. The discoloration is due to the production and secretion of substances which are a by-product of the death of food-storage cells. As new wood, that is sapwood, is formed to the outside of the tree stem, additional interior sap-wood adjacent to the heartwood zone is converted to heartwood. Some trees do not form discolored heartwood upon the death of... 

In addition to the major cell wall components of cellulose, hemicellulose and lignin, wood contains varying amounts of substances termed extractives. The term extractives includes a wide range of chemical types and a very large number of individual compounds. Some of the major chemical types are 1) Terpenes and related compounds, 2) Fatty acids, 3) Aromatic compounds and 4) Volatile oils. Species differ widely in the type and amount of extractives present. Also there is considerable variation in the distribution of extractives throughout the wood of individual trees. Although some extractives are found in sapwood, the heart-wood usually contains the largest amount. 

Fig. 1.1 Induced anatomical defense responses in Norway spruce. (A, B) Formation of a ring of new, traumatic resin ducts (TD, arrowheads) in the xylem of 2-year-old Norway spruce saplings after application of methyl jasmonate. A large cortical resin duct (CD) can be observed in the phloem, but these ducts do not appear to respond to methyl jasmonate treatment. (C) Normal phloem and sapwood anatomy of an older tree, with concentric rings of polyphenolic parenchyma cells (PP) in the phloem above the cambium (X) and normal wood below. (D) After treatment with methyl jasmonate or fungal infection the PP cells increase greatly in size and traumatic resin ducts (arrowheads) forms in the wood.

Heartwood - wud (1810) n. The wood extending from the pith to the sapwood, the cells of which no longer participate in the life processes of the tree. Heartwood may be infiltrated with gums, resins and other materials, which usually make it darker and more decay resistant than sapwood. 

Heartwood Primary xylem Heartwood is made of remaining dead cells from ancient sapwood. It forms the strongest part of the tree providing strength and support. It yields lumber and pulp. 

The response strategy of the tree is twofold first, to close the wound with new callus tissue, thus re-establishing that unbroken layer of living tissue and, second, to develop a reaction layer in the uninjured sapwood tissue adjacent to the damaged cells. The rate of wound closure and the production of the reaction zone are dependent not only upon the size of the wound but also upon the growth rate or vigor of the host. 

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