Secondary growth refers to the increase in the thickness or girth of plant organs due to the activity of lateral meristems, namely the vascular cambium and cork cambium. It is a characteristic feature of most dicotyledonous plants and gymnosperms. Secondary growth results in the formation of secondary tissues that provide mechanical strength, protection, and enhanced conducting capacity to the plant. While secondary growth is common in dicot stems and roots, it is generally absent in monocotyledonous plants.
Secondary Growth in Dicot Root
Secondary growth in dicot roots begins after the completion of primary growth and usually occurs in the mature region of the root. Unlike the dicot stem, a continuous vascular cambium is not present initially in the primary root. During secondary growth, cambial strips present between the primary xylem and primary phloem, along with certain pericycle and conjunctive tissue cells that regain meristematic activity, form a complete cambial ring.
1. Formation of Vascular Cambium
- The first step in secondary growth is the formation of a cambial ring.
- The cells of the conjunctive tissue located below the phloem strands become meristematic and form strips of cambium.
- Simultaneously, the pericycle cells situated opposite the protoxylem poles also become meristematic.
- These newly formed cambial strips join together to form a continuous but initially wavy ring of vascular cambium.
2. Activity of Vascular Cambium
- Once the cambial ring is established, it starts dividing actively.
- Cells produced towards the inner side differentiate into secondary xylem, while those produced towards the outer side differentiate into secondary phloem.
- The cambium produces much larger quantities of secondary xylem than secondary phloem. As a result, the root increases considerably in diameter.
- The continuous production of secondary vascular tissues pushes the primary xylem towards the centre and the primary phloem towards the outer side.
- The older secondary phloem and primary phloem gradually become crushed due to pressure exerted by newly formed tissues, whereas the primary and secondary xylem remain functional for a longer period.
3. Formation of Medullary Rays
- Some cambial cells function as ray initials and produce parenchymatous cells on both sides.
- These cells form broad medullary rays that extend radially through the secondary vascular tissues.
- Medullary rays facilitate the lateral transport of water, minerals, and food materials and also serve as storage tissues.
4. Formation of Periderm
- As secondary growth progresses, the outer tissues of the root are subjected to increasing pressure.
- Consequently, the epidermis and cortex may become stretched and eventually rupture.
- To provide protection, certain cells of the pericycle become meristematic and form the cork cambium or phellogen.
- The cork cambium produces cork (phellem) towards the outer side and secondary cortex (phelloderm) towards the inner side.
- Together, the cork, cork cambium, and secondary cortex constitute the periderm, which replaces the epidermis as the protective outer covering of the root.
Periderm = Phellem (Cork) + Phellogen (Cork Cambium) + Phelloderm (Secondary Cortex)
Lenticels
Lenticels may develop in the periderm to facilitate gaseous exchange between internal tissues and the external environment. These structures are important for aeration in older roots.
Annual Rings
Annual rings are generally not prominent in roots because underground environmental conditions remain relatively stable throughout the year and do not produce distinct seasonal variations in cambial activity.
Secondary Growth in Monocot Roots
- In most monocotyledonous plants, secondary growth is absent because vascular cambium is not formed.
- The vascular bundles remain closed and therefore cannot produce secondary xylem or secondary phloem.
- As a result, monocot roots generally do not show a significant increase in girth through normal secondary growth.
- However, certain monocot plants such as Dracaena, Agave, Yucca, and Aloe exhibit an unusual increase in thickness due to the formation of accessory cambia.
- This type of growth differs from normal secondary growth and is known as anomalous secondary growth.
Significance of Secondary Growth
- It increases the girth and mechanical strength of stems and roots.
- It enables plants to support larger crowns, branches, leaves, flowers, and fruits.
- It produces additional xylem and phloem, thereby improving the transport of water, minerals, and food materials.
- It forms protective tissues such as cork and bark, which protect the plant against mechanical injury, pathogens, excessive heat, and water loss.
- It contributes to the formation of wood, which is economically important for construction, furniture making, paper production, and fuel.
- Thus, secondary growth is a vital process that allows woody plants to attain large sizes, survive for many years, and efficiently conduct water and nutrients throughout the plant body.