Plant Anatomy: Secondary Growth

Secondary Growth in Woody Plants

• Conifers and woody dicot shrubs and trees exhibit secondary growth.
• The stems and roots of woody plants grow in diameter (girth) due to the activity of the vascular cambium (secondary tissue).

Vascular Cambium

• The vascular cambium develops between the xylem and phloem.
• It produces new xylem inside and new phloem outside.

Cork Cambium

• Cork cambium is also a secondary tissue that gives rise to the periderm.

Zones of Developing Root

• The root cap covers the tip of the root.
• There are three zones: Central zone (CZ), Peripheral zone (PZ), and Rib zone (RZ).
• The Central zone (CZ) contains stem cells.
• The Peripheral zone (PZ) produces leaf primordia.
• The Rib zone (RZ) lies below the CZ and generates the central tissue.

Root Growth

• After cells start to elongate and mature, no further extension takes place, and the root is stationary for the rest of its life.

Intercalary Meristems

• Intercalary meristems are found at nodes of bamboo and allow for rapid stem elongation.
• They are also found in grass leaf blades and allow for rapid regrowth of damaged leaves.

Plant Meristems

• Plant growth is limited to certain regions where cells retain the capacity for continued cell division.
• Meristems are sites of cell division and growth in plants.

Apical Meristem

• Located under and behind the root cap.
• Produces cells that give rise to the primary body of the plant.

Zones of the Root

Zone of Cell Division

• Area of active cell division.
• Gives rise to the three primary meristems:
  • Protoderm (gives rise to epidermis).
  • Procambium (produces xylem and phloem).
  • Ground meristem (produces cortex and pith).

Quiescent Center

• Area where cells divide more slowly.
• Located between the apical meristem and the root cap.

Zone of Cell Elongation

• Cells stretch and lengthen in this zone.
• Small vacuoles coalesce and fill with water, causing cellular expansion.

Zone of Maturation

• Elongating cells complete their differentiation in this zone.
• Recognizable by the presence of root hairs extending into the soil.

Intercalary Meristem

• Found only in monocot stems.
• No further extension takes place after cells start to elongate and mature.

Root Cap

• Covers the tip of the root.
• Protects the root cells from drying out and facilitates water movement from the soil into the plant.

Meristem Classification

• Meristems are classified by their location in plants as:
  • Apical Meristems (AMs) at the ends of roots and stems
  • Lateral Meristems (LMs) in the vascular and cork cambia

Apical Meristems

• Located underneath and behind the root cap
• Produces cells that give rise to the primary body of the plant
• Consists of three zones:

  Zone of Cell Division

  • Area of active cell division
  • Quiescent center: an area where cells divide slowly

Organization of Root Apical Meristem

• Organizes to form three primary meristems:
  • Protoderm: gives rise to the epidermis
  • Procambium: produces xylem and phloem
  • Ground meristem: produces the cortex

Zone of Cell Elongation

• Cells stretch and lengthen as small vacuoles within the cytoplasm coalesce and fill with water
• Cellular expansion responsible for pushing the root cap and apical tip forward through the soil

Zone of Maturation

• Elongating cells complete their differentiation in this zone
• Recognized by the presence of numerous root hairs that extend into the soil as outgrowths of epidermal cells

Intercalary Meristems

• Found only in monocot stems
• Formed at specific sites called meristems

Nature of Plant Meristems

• Plant growth is limited to certain regions where cells retain the capacity for continued cell division
• Unlike animals, which have a generalized growth pattern