Secondary Growth

6.5 Secondary Growth

The growth of the roots and stems in length with the help of apical meristem is called the primary growth. Apart from primary growth most dicotyledonous plants exhibit an increase in girth. This increase is called the secondary growth. The tissues involved in secondary growth are the two lateral meristems: vascular cambium and cork cambium.

6.5.1 Vascular Cambium

The meristematic layer that is responsible for cutting off vascular tissues — xylem and phloem — is called vascular cambium.

Formation of Cambial Ring

In the young stem, vascular cambium is present as a single layer between xylem and phloem within each vascular bundle (fascicular cambium). Later, a few cells of the medullary rays adjoining the fascicular cambium become meristematic and form the interfascicular cambium. Thus, a continuous ring of cambium is formed.

Activity of Cambial Ring

The cambial ring becomes active and begins to cut off new cells, both towards the inner and the outer sides. The cells cut off towards pith mature into secondary xylem and the cells cut off towards periphery mature into secondary phloem. The cambium is generally more active on the inner side than on the outer. As a result, the amount of secondary xylem produced is more than secondary phloem, and soon a ring of secondary xylem surrounds the pith.

The primary and secondary phloems get gradually crushed due to the continued formation and accumulation of secondary xylem. The primary xylem however remains more or less intact, in or around the centre. At some places, the cambium forms a narrow band of parenchyma, which passes through the secondary xylem and the secondary phloem in the radial directions. These are the secondary medullary rays.

6.5.2 Spring Wood and Autumn Wood

The activity of cambium is under the control of many physiological and environmental factors. In temperate regions, the climatic conditions are not uniform throughout the year. In the spring season, cambium is very active and produces a large number of xylary elements having vessels with wider cavities. The wood formed during this season is called spring wood or early wood. In winter, cambium is less active and forms fewer xylary elements that have narrow vessels, and this wood is called autumn wood or late wood.

The spring wood is lighter in colour and has a lower density whereas the autumn wood is darker and has a higher density. The two kinds of woods that appear as alternate concentric rings, constitute an annual ring. Annual rings seen in a cut stem give us an estimate of the age of the plant.

6.5.3 Heartwood and Sapwood

In old trees, the greater part of the secondary xylem is dark brown due to deposition of organic compounds like tannins, resins, oils, gums, aromatic substances and essential oils. These substances make the wood hard, durable and resistant to attacks of microorganisms and insects. This region comprises dead elements with highly lignified walls. It is called the heartwood or duramen. The heartwood does not conduct water but it gives mechanical support to the stem.

The peripheral region of the secondary xylem, is lighter in colour and is known as the sapwood or alburnum. It is involved in the conduction of water and minerals from root to leaf.

6.5.4 Cork Cambium (Phellogen) and Periderm

As the stem continues to increase in girth due to the activity of vascular cambium, the outer cortical and epidermis layers get broken and need to be replaced to provide new protective cell layers. Hence, soon a new meristematic tissue called cork cambium or phellogen develops, usually in the cortex region. Phellogen is a couple of layers thick. It is made of narrow, thin-walled, and nearly rectangular cells.

Phellogen cuts off cells on both sides. The outer cells differentiate into cork (phellem) while the inner cells differentiate into secondary cortex (phelloderm). The cork is impervious to water due to suberin deposition in the cell wall. The cells of secondary cortex are parenchymatous. Phellogen, phellem, and phelloderm are collectively known as periderm.

Due to activity of cork cambium, pressure builds up on the remaining layers peripheral to phellogen and ultimately these layers die and slough off. Bark is a non-technical term that refers to all tissues exterior to the vascular cambium, therefore including secondary phloem, primary phloem, cortex (if any), periderm and phelloderm. Bark formed early in the season is called early or soft bark; towards the end of the season, late or hard bark is formed.

At certain regions, the phellogen cuts off closely arranged parenchymatous cells on the outer side instead of cork cells. These parenchymatous cells soon rupture the epidermis, forming a lens-shaped openings called lenticels. Lenticels permit the exchange of gases between the outer atmosphere and the internal tissues.

6.5.5 Secondary Growth in Roots

In dicot roots, the vascular cambium is completely secondary in origin. It originates from the tissue located just below the phloem bundles, and a portion of pericycle tissue, above the protoxylem forming a complete and continuous wavy ring, which later becomes circular. Further activity is similar to that already described in stems.

The secondary growth also occurs in roots of gymnosperms but is generally absent in monocots, which include grasses and palms. Some monocots like dragon tree (Dracaena) and date palm show anomalous secondary growth.

Worked Examples