Secondary Growth

Secondary growth is the formation of additional layers of secondary tissues, brought about by the activity of vascular cambium and cork cambium, serves to augment the plant’s girth, thickness, or diameter. The secondary tissues produced by cambium activities during this growth phase are referred to as secondary tissues. The specifics of secondary growth differ across various plant categories and components.

Primary growth initiates right after seed germination and is driven by the action of primary meristems. This type of growth leads to the elongation of the plant and its lateral attachments. On the other hand, secondary growth primarily contributes to an increase in the plant’s girth or thickness.

The thickness of stems and roots increases through the activity of secondary lateral meristems, specifically the vascular cambium and cork cambium. This type of additional growth is commonly seen in gymnosperms and dicot plants, and occasionally in certain monocots like Dracaena. It does not occur in the leaves.

Secondary Growth in Dicot Stem

Secondary growth primarily occurs as a consequence of the functions carried out by vascular cambium and cork cambium. The specifics of secondary growth differ across various plant categories and components but can be studied under following headings;

• Secondary growth in stellar region
• Secondary growth in extra-stellar region

Secondary Growth in Stellar Regions

The sequence of events for secondary growth, facilitated by the actions of vascular cambium, occurs as follows;

• Formation of cambium ring
• Formation of secondary tissues
• Formation of annual rigs
• Formation of heartwood and sapwood

Formation of cambium ring

Intra-fascicular cambium: Within a primary vascular bundle, a strip of cambium known as intra-fascicular cambium exists between the xylem and phloem. As secondary growth occurs, specific cells within the medullary rays, positioned at the intra-fascicular cambium level, become meristematically active.

Inter-fascicular cambium: The cells of intra-fascicular cambium give rise to a cambium strip situated between vascular bundles, referred to as inter-fascicular cambium.

Cambium ring: These inter-fascicular cambium strip joins with the intra-fascicular cambium on both sides, resulting in the formation of a continuous cambium ring.

Formation of secondary tissues

Subsequently, the complete cambium ring becomes activated, and the cambium cells initiate division along a tangential plane. The cambium ring functions collectively as a meristem, contributing secondary tissues to both the inner and outer regions.

The cambium ring comprises two distinct cell types: elongated, spindle-shaped fusiform initials and shorter, isodiametric ray initials.

Fusiform initials: Positioned between the xylem and phloem within the vascular bundle, the fusiform initials undergo division to generate secondary phloem on the outer side and secondary xylem on the inner side. The fusiform initials exhibit a higher degree of activity toward the inner region compared to the outer region.

As a result, they produce more secondary xylem on the inner side and a smaller amount of secondary phloem on the outer side. Consequently, the primary xylem and primary phloem are displaced towards the pith and periphery, respectively, due to the formation of secondary xylem and secondary phloem.

Ray initials: In parallel, the ray initials, located amidst the vascular bundles, divide to give rise to vascular rays or secondary medullary rays on both sides.

The fusiform initials display greater activity compared to the ray initials. Consequently, a larger quantity of secondary vascular tissues is formed in contrast to secondary medullary rays. In this scenario, the secondary vascular tissues exert pressure on the secondary medullary rays, causing them to condense and form vascular rays.

Vascular rays: It is located amidst the secondary xylem are termed as wood rays or xylem rays, while the portion of vascular rays found within the secondary phloem is referred to as phloem rays. In the majority of angiosperm tree species, the ray parenchyma is comparatively limited, resulting in harder wood referred to as pycnoxylic wood.

The cambium remains active for extended durations. Typically, the production of inner secondary tissue surpasses that of outer secondary tissue causing the cambium ring to shift towards the periphery.

Formation of annual rigs

Spring wood: In temperate zone trees, the activity of the cambium ring experiences variations throughout the year. The cambium ring displays heightened activity during favorable periods like spring and early rainy seasons, resulting in the formation of broader vessels containing increased amounts of secondary xylem referred to as spring wood or early wood.

Autumn wood: In the less favorable winter season, the cambium ring is less active, leading to the creation of smaller vessels containing secondary xylem, known as autumn wood or late wood.

Annual ring: These spring wood and autumn wood manifest as a concentric ring within a single year, forming what is termed an annual ring or growth ring. These rings alternate according to the season and they are of lighter and darker shades. Spring wood ring and one autumn wood ring together represent a year which accumulate year by year.

Formation of heartwood and sapwood

Within mature trees, the secondary xylem differentiates into two distinct regions known as heartwood and sapwood.

Heart wood: Heart wood also referred to as duramen, constitutes the darker central portion of the wood and is devoid of physiological activity. It contains accumulated extractive substances like oils, resins, gums, and tannins within both the cell walls and lumens. Heartwood serves as a source of mechanical support for the stem, it doesn’t participate in water and mineral conduction.

Sap wood: the outer section of the wood, which appears lighter in color and lacks extractives in its cell walls and lumens, is termed sapwood or alburnum. Even though sapwood contains some living cells, it is notably weaker and less durable compared to heartwood. sapwood plays a vital role in facilitating the upward transport of water and mineral nutrients from the root to the leaf, as well as in food storage.

Secondary Growth in Extra Stellar Regions

Extra-stellar secondary growth occurs due to specific lateral meristem called the cork cambium. The cork cambium is necessary to overcome the damage (rupture) of the epidermis and cortex caused due to increasing girth. It can be studied under following headings;

• Formation of Cork Cambium or Phellogen
• Formation of Lenticels

Formation of Cork Cambium or Phellogen

The outermost layer of cortical cells, occasionally including deeper cortical or pericycle cells, undergoes a revival of meristematic activity, leading to the development of cork cambium, also known as phellogen.

The phellogen cells divide both outwardly and inwardly, giving rise to secondary tissues. The secondary tissue formed on the inner side of the phellogen is termed secondary cortex or phelloderm. The phellogen generates cork cells or phellem on its outer side.

The composite structure encompassing phelloderm, cork cambium, and cork is identified as periderm.

The cork cells consist of deceased, tightly arranged rectangular cells featuring cell walls enriched with suberin. The cork cambium is more active toward the outer surface compared to the inner side.

There is a greater production of phellem on the outer side than phelloderm on the inner side.

The cork cells serve multiple functions, such as preventing water loss through evaporation, safeguarding interior cells against detrimental microorganisms, mechanical harm, and adverse conditions.

Formation of Lenticels

In specific locations where stomata were previously present on the epidermis, the phellogen generates air-passing openings in the plant’s bark, instead of producing cork cells. These openings are known as lenticels.

Lenticels form where the phellogen’s activity is higher compared to other areas, resulting in the development of loosely arranged, thin-walled, rounded cells with numerous spaces between them, as opposed to the thick-walled, suberized cork cells. These loosely arranged cells are termed complementary cells, which may have either suberized or unsuberized cell walls.

As the quantity of these cells increases, they apply pressure on the epidermis, creating small elevated spots that rupture the epidermal layer. Lenticels serve as conduits for gas exchange, facilitating lenticular transpiration and the release of harmful compounds.

Significance of Secondary Growth

• Secondary growth increases the girth (diameter) of stem, hence provide mechanical support for aerial growth of the plant.
• It adds new conducting tissues for the replacement of old and non-functioning ones required for meeting increased demand for long distance transport of water and minerals.
• It produces corky bark around the tree trunk which protects internal tissues from environmental adversities and pathogens.
• It helps to determine the age of plant by counting the number of annual rings(dendrochronology).
• Lenticels produced by secondary growth help in exchange of gases and transpiration.
• Secondary growth is responsible for changing sap wood to heart wood during the course of time.