Threats to Biodiversity

What is heartwood and sapwood?
Answered by Discovery Channel
  • Discovery Channel

    Discovery Channel

  1. The cells of a tree are composed of cellulose and lignin -- the lignin binds and hardens the cellulose molecules. There are two kinds of tree cells: water-conducting cells and sugar-conducting cells. The sugar-conducting cells are located outside the former.

    Water-conducting cells make up the tree's xylem, which transports water from the roots to all the branches and leaves. Sugar-conducting cells make up a tissue called phloem, which carries nutrients from the leaves to the rest of the tree.

    Sandwiched between the xylem and phloem are stem cells called the cambium. These cells generate new phloem cells on the outside and new xylem cells on the inside. As the tree grows, the older xylem cells are closer to the center of the trunk. These cells are called the heartwood, and they are the strongest part of a tree. The newer xylem is called sapwood.

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