Unit-IV: Vascular Cambium and Wood

Cytodifferentiation is the process by which specialized cells like tracheary elements (xylem) and sieve elements (phloem) are formed.

• Tracheary Elements: Involves the development of secondary cell walls with lignin deposition and the eventual programmed death of the cell to form a hollow tube for water transport.
• Sieve Elements: Involves the loss of the nucleus and other organelles to facilitate the efficient transport of organic nutrients while remaining living cells supported by companion cells.

These structures facilitate communication and transport between adjacent plant cells.

• Pits: Thin areas in the secondary cell wall where only the primary wall and middle lamella remain, allowing for the movement of water and solutes between xylem vessels.
• Plasmodesmata: Microscopic channels that traverse the cell walls of plant cells, enabling direct cytoplasmic communication and transport of signaling molecules between cells.

The vascular cambium is a lateral meristem responsible for increasing the girth of the plant.

• Structure: Composed of two types of initials: Fusiform initials (give rise to longitudinal xylem and phloem) and Ray initials (give rise to vascular rays).
• Function: Produces secondary xylem (wood) toward the inside and secondary phloem toward the outside.
• Seasonal Activity: In temperate regions, the cambium is more active in spring (producing wide-vessel springwood) and less active in autumn (producing narrow-vessel latewood).

Secondary growth results in an increase in thickness in dicots and gymnosperms.

• In Stem: Triggered by the formation of a continuous cambial ring from intrafascicular and interfascicular cambium.
• In Roots: The cambium originates from tissues located just below the phloem bundles and above the protoxylem, eventually forming a wavy and then circular ring.

Some plants exhibit growth patterns that deviate from the normal "inner xylem, outer phloem" arrangement.

• Causes: Can be due to an abnormal position of the cambium, abnormal activity of a normal cambium, or the formation of accessory cambium.
• Examples: Found in plants like Boerhavia, Bignonia, and Dracaena.

As wood ages, its structure and function change.

Tyloses

Outgrowths from adjacent parenchyma cells into the lumen of xylem vessels through pits. They block the vessels in heartwood, preventing water flow and protecting against pathogens.

Growth Rings

A pair of springwood and autumnwood produced in one year constitutes an annual ring or growth ring.

Dendrochronology

Definition: The science of determining the age of a tree and analyzing past climate conditions by counting and studying annual growth rings.