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Stems and Plant Growth Basics Shoots vs. Roots • Stems are part of the shoot system (stems, leaves, flowers) • The shoot system depends on the roots for water and minerals, and to store excess sugar created by photosynthesis Anatomy of a Stem • Node: area where leaf branches off from stem • Internode: area in between nodes • Apical meristem: terminal bud where upward growth in concentrated • Axillary bud: site of new branches • Petiole: area connected leaf to stem Anatomy of a Stem • Leaf scar: area where leaf has fallen from stem • Terminal bud scar: remnant of previous year’s terminal bud; marks one year of growth Terminal vs. Axillary Buds • The growth of young shoots is concentrated at the terminal bud, where cells are dividing rapidly • The terminal bud is dominant over the axillary buds in young plants, keeping them dormant • We can prune/pinch plants to influence shape – Trimming terminal bud increases width – Trimming axillary buds increases height Inside a Stem • Young stems are made of xylem, phloem, a pith, vascular cambium, and epidermis – Xylem: transports water from roots to leaves – Phloem: transports sugar from leaves to roots – Pith: center of stem; functions in storage – Vascular cambium: layer of cells that makes new xylem and phloem – Epidermis: protective coating Inside a Stem • Dicots have a large central pith, with vascular bundles around the edges (a vascular cylinder) • Monocots have vascular bundles randomly dispersed throughout the pith Why do plants grow forever? • Plants can grow throughout their lives because of meristems • Meristems are bundles of stem cells that make new cells throughout a plant’s life – Apical meristems: responsible for primary plant growth (increase in height) – Lateral meristems: responsible for secondary plant growth (increase in diameter) Primary Plant Growth • Apical meristems are located at the tips of roots and stems • Apical meristems give rise to the epidermis, vascular tissue (transport), and ground tissue (storage) Secondary Plant Growth • Secondary growth increases plant diameter • Lateral meristems cause secondary growth – Vascular cambium – Cork cambium Vascular Cambium • The vascular cambium is located between layers of xylem and phloem • The vascular cambium makes secondary xylem and phloem (wood and bark) Cork Cambium • The cork cambium makes cork, a tough outer coating that replaces the epidermis on stems and roots • This is what we think of as ‘bark’, but technically bark includes secondary phloem, the cork cambium, and cork