stems

• Stem Functions


  • Support leaves and reproductive structures
  • Conduct water, dissolved minerals, carbohydrates
  • Produce new living tissues at apical meristems
  • Produce new living tissues at lateral meristems (secondary growth)
• Variation in Stems


  • Herbaceous
  • Woody
  • Primary Growth
  • Secondary Growth
• Herbaceous stems


  • Lack secondary growth
  • Monocots usually lack secondary growth
  • Secondary growth differs from the typical pattern
• Dicot stems


  • Have vascular bundles arranged in a circle (in cross section)
  • Have a distinct cortex and pith
• Monocot stems


  • Have scattered vascular bundles
  • Have ground tissue instead of distinct cortex and pith
• Tissues in Herbaceous Stems

  • Epidermis
  • Vascular tissues (Xylem, Phloem)
  • Storage tissues (Cortex, Pith)
  • Ground tissue
• Stems have nodes and internodes, leaves and buds, while roots have root caps and root hairs
• Node


  Area on a stem where one or more leaves is attached
• Internode


  Stem area between two successive nodes
• Bud


  An undeveloped shoot that contains an embryonic meristem, may be terminal (at tip of stem) or axillary (on side of stem)
• Herbaceous roots possess an endodermis and pericycle, while stems lack a pericycle and rarely have an endodermis
• Vascular bundles in a sunflower stem are arranged in a circle
• Each vascular bundle in a sunflower stem is "capped" by a batch of fibers for additional support
• Vascular Cambium


  A lateral meristem that produces secondary xylem (wood) to the inside and secondary phloem (inner bark) to the outside
• Vascular cambium is not initially a solid cylinder of cells, but becomes continuous when production of secondary tissues begins
• Certain parenchyma cells between vascular bundles retain ability to divide and connect to vascular cambium cells in each vascular bundle to form a complete ring of vascular cambium
• Dividing Vascular Cambium


  1. First division forms a xylem cell and a vascular cambium cell

  2. Second division forms a phloem cell

  3. Subsequent divisions continue to form xylem and phloem cells
• Cork Cambium


  A lateral meristem that produces cork parenchyma to the inside and cork cells to the outside
• Cork cambium and the tissues it produces make up the outer bark of a woody plant
• Development of Woody Eudicot Stem


  1. Vascular cambium arises in the parenchyma between the vascular bundles

  2. Vascular cambium begins to divide, forming secondary xylem on the inside and secondary phloem on the outside

  3. Vascular cambium produces more secondary xylem than secondary phloem
• Lenticels are pores that promote gas exchange of oxygen, carbon dioxide, and water vapor
• Annual rings in tree trunks are formed by the vascular cambium producing alternating layers of light springwood and darker summerwood
• Heartwood and sapwood are different regions of the secondary xylem in tree trunks
• Stem Modifications


  • Rhizome
  • Tuber
  • Bulb
  • Corm
  • Stolon
  • Tendril
  • Cladophyll
• Rhizome


  A horizontal underground stem that often serves as a storage organ and a means of sexual reproduction
• Tuber


  The thickened end of a rhizome that is fleshy and enlarged for food storage
• Bulb


  A rounded, fleshy underground bud that consists of a short stem with fleshy leaves
• Corm


  A short, thickened underground stem specialized for food storage and asexual reproduction
• Stolon


  An aerial horizontal stem with long internodes; often forms buds that develop into separate plants
• Tendril


  A slender threadlike appendage of a climbing plant, often growing in a spiral form, that stretches out and twines around any suitable support
• Cladophyll


  Shoot systems in which leaves do not develop; rather, the stems become flattened and assume the photosynthetic functions of the plant