Askjagden's Guide to Biology: Stems
One of the three principal organs of the plant, stems transport water and nutrients, support and defend the plant, produce leaves, branches, and flowers, and hold leaves up. Because they produce new parts of the cells, stems have places called nodes on them, in which leaves are attached, and buds, which are undeveloped tissue that can grow into new things. Stems can be classified as woody or herbaceous.
Here's one characteristic of stems that helps distinguish between monocots and dicots. In monocots, vascular bundles (bundles that are made up of xylem and phloem) are scattered throughout the stem. In dicots, vascular bundles are arranged cylindrically. Plus, in dicots, there is ground tissue in the center of the vascular tissue. That ground tissue is called pith.
How do stems grow? Stems grow in two ways: vertically or horizantally. Vertical growth is called primary growth, while horizantal growth is called secondary growth. Primary growth is possible through the apical meristems, which are the only parts of the plant that grows through cell division. The apical meristems, remember, are located at the ends of the plant. Secondary growth works through horizantal meristematic tissue called cork cambium and vascular cambium. The cork cambium produces the epidermis, while the vascular cambium produces vascular tissue.
Remember what I said about woody plants and vascular cambium? Vascular cambium helps produce wood, which is mostly made up of xylem. As secondary growth goes on, the center of the woody plant becomes heartwood. Heartwood no longer functions as it is supposed to. Heartwood is surrounded by wood called sapwood, which is more workable than heartwood. Bark is made up of vacular tissue; specifically, bark is made up of cork and cork cambium. Cork is made up of vascular phloem that no longer works. Bark helps to protect the plant.
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