Plant Physiology - Musiyenko, M. M. 2001

Physiology of Plant Growth and Development
Secondary Growth

Lateral Meristems are responsible for secondary growth, which results in the thickening of stems and roots. There are two types of lateral meristems: the vascular cambium, which gives rise to Vascular Tissues, and the phellogen, or cork cambium, which develops later. Secondary growth causes the stem and ROOT to thicken through The production of secondary xylem — wood and secondary phloem — bast, both of which are characteristic of trees and shrubs.

Initially, the vascular cambium is located between the primary xylem and primary phloem of the vascular bundles. Immediately after Cell Differentiation is complete, it becomes active, driving secondary stem growth in woody plants. Cambial activity is stimulated by phytohormones such as Auxins and Gibberellins, supplied by buds and leaves. As the stem thickens, the circumference of the cambial layers also increases. This is made possible by the radial division of cambial Cells. Primary medullary rays are formed, extending from the pith to the cortex. They serve as a vital link between the pith and the bark, facilitating the radial transport of Water and mineral salts through the xylem and organic nutrients through the phloem. Nutrients are stored here, which is crucial during dormant periods, such as over winter. In temperate climates, growth resumes every spring. Characteristically, the latewood formed at the end of the growing season contrasts sharply with the earlywood of the following year directly adjacent to it. As a result, annual rings are formed, clearly visible in cross-section.

The gradual increase in stem circumference affects the protective tissue layers. As the stem thickens, the epidermis ruptures and is replaced by cork, which forms just beneath the epidermis through The activity of another lateral meristem — the phellogen. Cork cells are deposited outward, while one or two layers of parenchyma are formed inward. Together, these layers constitute the phelloderm. The phellogen, cork, and phelloderm make up the periderm. Cork cells become impregnated with suberin, rendering them impermeable to water and gases. They gradually die, losing their living contents, and become filled with air, resins, or Tannins. They envelop the tree trunk, protecting it against desiccation, infections, and mechanical damage.

In plants that exhibit secondary stem growth, secondary root growth is also common. The Development of secondary xylem, secondary phloem, and medullary rays in roots is fundamentally similar to their development in the stem.