Advanced Plant Anatomy and Secondary Thickening

Plants display two distinct phases of development: primary and secondary growth. During primary growth, shoot apical meristems (SAMs) and root apical meristems (RAMs) are responsible for the aboveground and underground organ, respectively. After the primary vascular system is established, fascicular cambium located at the center of primary vascular bundles undergoes extension into the interfascicular region, forming a ring of vascular cambium.

Vascular Cambium

Vascular cambium is a cylindrical secondary meristem whose activity gives rise to secondary growth. It forms the wood and the inner bark of the tree and is responsible for thickening the plant. Vascular cambium is a ring of meristematic cells that produces secondary xylem (wood) along the inner edge and secondary phloem (secondary vascular tissues) along the outer edge. The vascular cambium is a lateral meristem that originates from procambium, which has not completely differentiated during the formation of primary xylem and primary phloem.

Bark Structure

The bark of a tree is composed of outer cork cambium (phellogen), phellem (outer cork cells), and phelloderm (inner secondary cortex). Cork cambium is formed in response to the crushing of outer layers such as cortex cells and epidermis during secondary growth. The differentiation of cork cambium cells leads to the formation of phellem and phelloderm, which together make up the periderm, protecting the plant from biotic and abiotic stresses.

Vascular Tissue

Vascular tissues, including xylem and phloem, are conducting and supporting tissues found in the secondary growth of plants. Xylem, derived from vascular cambium, is responsible for water transport and provides structural support, while phloem transports nutrients and hormones. The formation of these tissues is regulated by various signals such as auxin, cytokinins, brassinosteroids, gibberellins, ethylene, TDIF peptide, and their cross-talk.

Wood Formation

Wood, or secondary xylem, is derived from vascular cambium and undergoes a series of developmental processes, including initiation of vascular stem cells, proliferation, and differentiation. The initiation of vascular stem cells is regulated by hormones and peptides, which play a crucial role in determining the structure and function of the wood.

Secondary Growth

Secondary growth is a characteristic feature of dicotyledonous plants, with most monocotyledons lacking this feature. Secondary growth in plants is responsible for the increase in stem thickness and is due to the activity of the lateral meristems, which are absent in herbs or herbaceous plants. The secondary growth of plants is characterized by the formation of secondary xylem and phloem, which form the cambial ring in plants.

In conclusion, vascular cambium plays a vital role in the secondary growth of plants, forming the wood and inner bark of trees and responsible for thickening the plant. The bark structure, vascular tissues, and wood formation are all regulated by the activity of the vascular cambium and are essential for the growth and development of plants.