Secondary Growth

Secondary Growth

Primary growth refers to the lengthening of roots and stems with the help of the apical meristem. Most dicotyledonous plants expand in girth in addition to main growth. This is referred to as secondary growth. The two lateral meristems, vascular cambium and cork cambium, are engaged in secondary growth.

A. Vascular Cambium:

Vascular cambium is the meristematic layer responsible for cutting off vascular tissues – xylem and phloem. It appears in patches as a single layer between the xylem and phloem in the young stem. It eventually forms a complete ring.

i) Formation of cambial ring:

The intrafascicular cambium is the cambium cells found between the primary xylem and primary phloem in dicot stems. The interfascicular cambium is formed when the cells of medullary rays next to this intrafascicular cambium become meristematic. As a result, a continuous ring of cambium forms.

ii) Activity of the cambial ring:

The cambial ring becomes active and starts cutting off new cells from both the inside and outside. Secondary xylem develops from cells cut off towards the pith, while secondary phloem develops from cells cut off towards the periphery. The inner side of the cambium is often more active than the outer side. As a result, secondary xylem is formed in greater quantities than the secondary phloem, forming a compact mass. The persistent development and accumulation of secondary xylem gradually crushes the primary and secondary phloems. The principal xylem, on the other hand, is mostly intact at or near the centre.The cambium sometimes produces a narrow band of parenchyma that runs in radial directions through the secondary xylem and secondary phloem. These are the secondary medullary rays.

iii) Springwood and autumn wood:

Cambium activity is influenced by a variety of physiological and environmental variables. Climate conditions in temperate zones are rarely consistent throughout the year. The cambium is particularly active in the spring and produces a significant number of xylary components with larger vessels. Springwood, also known as earlywood, is formed during this season. Autumn wood or latewood is formed when the cambium is less active in the winter and produces fewer xylary components with thin channels. Springwood is lighter in color and denser, whereas fall wood is darker and denser. An annual ring is made up of two types of wood that appear as alternate concentric rings.Annual rings in a cut stem can be used to estimate the age of a tree.

iv) Heartwood and sapwood:

The deposition of organic components such as tannins, resins, oils, gums, aromatic chemicals, and essential oils in the middle or innermost layers of the stem causes the majority of secondary xylem in ancient trees to be dark brown. These compounds make it tough, long-lasting, and resistant to bacteria and insects. Heartwood is a zone made up of dead components with heavily lignified walls. The stem is supported mechanically by the heartwood, which does not conduct water. Sapwood refers to the lighter-colored portion of the secondary xylem. It helps to transport water and minerals from the root to the leaf.

B.Cork Cambium:The outer cortical and epidermal layers break down as the stem grows in girth due to vascular cambium activity, and they must be replaced to create new protective cell layers. As a result, additional meristematic tissue is known as cork cambium or phellogen forms, usually in the cortical region, sooner or later. Phellogen is made up of several layers. It is made up of cells that are tiny, thin-walled, and practically rectangular. Both sides of the cell are cut off by Phellogen. Inner cells differentiate into secondary cortex or phelloderm, whereas exterior cells differentiate into cork or phellem. Due to suberin deposition in the cell wall, the cork is impenetrable to water. Secondary cortical cells are parenchymatous. The periderm is the combination of phellogen, phellem, and phelloderm.

Pressure goes up on the remaining layers peripheral to phellogen due to cork cambium activity, and these layers eventually die and peel off. Secondary phloem is included in the phrase "bark," which is a non-technical term that refers to all tissues outside of the vascular cambium. Periderm and secondary phloem are two forms of tissue that makeup bark. Early or soft bark refers to bark that forms early in the season. Late or hard bark develops toward the end of the season. Instead of cork cells, the phellogen breaks off tightly packed parenchymatous cells on the outer surface in some areas. These parenchymatous cells quickly break the epidermis, generating lenticels, which are lens-shaped holes.Lenticels allow gas exchange between the outside atmosphere and the stem's internal tissue. Most woody trees have these.

C. Secondary Growth in Roots:

The vascular cambium in dicot roots is entirely secondary in origin. It develops from tissue right beneath the phloem bundles, a piece of pericycle tissue, and above the protoxylem, forming a complete and continuous wavy ring that eventually becomes circular. The next events are similar to those outlined before for a dicotyledon stem. Gymnosperm stems and roots also experience secondary growth. Monocotyledons, on the other hand, do not have secondary growth. Secondary growth usually results in a thickening of the root diameter due to the addition of vascular tissue. When cells in the residual procambium and sections of the pericycle begin to make periclinal divisions, secondary growth begins. Periclinal divisions begin only in the pericycle cells opposite the xylem sites. The vascular cambium is formed by the inner layer of cells. The pericycle is the outer layer that is retained. Around the primary xylem, the vascular cambium is continuous. The periclinal division of the vascular cambium continues. If the daughter cells divide towards the core of the root, they become secondary xylem cells, and if they divide towards the outer surface of the root, they become secondary phloem cells.The periderm, which comes from the pericycle and replaces the epidermis, forms an outer protective layer on certain roots. The pericycle restores its meristematic nature and divides periclinallyonce more and thus forms the phellogen or cork cambium. To the outside of the plant, the cork cambium generates phellum cells (cork cells). At maturity, these cells are lifeless. They've been suberized, which makes the cells water-resistant. Phellum cells in cross-section are neatly organized into files. The phelloderm, a mature tissue made up of live cells, is likewise produced by the cork cambium.