Secondary Growth in Dicot Stem

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Here, I am posting a concise description of the secondary growth in dicot stem.

Schematic diagram of primary and secondary growth in dicot stem 
Image source: https://commons.m.wikimedia.org/wiki/File:Crescita_primaria_e_secondaria_del_fusto.png#file

The process of formation of extra layers or secondary tissues due to activity of vascular cambium and cork cambium for increasing girth or thickness or diameter of plant is called secondary growth. It takes place in stellar regions due to the activity of vascular cambium (see Figure 17) and in extra-stellar regions due to the activity of cork cambium (see Figure 21).

Secondary growth in stellar regions due to the activity of vascular cambium

Secondary growth due to activity of vascular cambium takes place by the formation of cambium ring, formation of secondary tissues, formation of annual rings or growth rings, and formation of heartwood and sapwood.

Formation of cambium ring

The dicot stem has a strip of cambium in between xylem and phloem in vascular bundles. This cambium is known as intrafascicular cambium. During secondary growth, some cells of medullary rays that are present at the level of intrafascicular cambium show meristematic activity to form the cambium between two adjacent vascular bundles, called interfascicular cambium. Intrafascicular cambium and interfascicular cambium join to form a cambium ring.

Formation of secondary tissues

Cambium ring acts as a meristem to add the secondary tissues both towards inner side and outer side. Cambium ring has two types of cells – elongated spindle-shaped fusiform initials and short isodiametric ray initials (see Figure 18). Fusiform initials divide to form secondary phloem towards outer side and secondary xylem towards inner side. The primary phloem and primary xylem are pushed towards periphery and centre by secondary phloem and secondary xylem, respectively. Ray initials divide to form vascular rays or secondary medullary rays on both sides. The activity of fusiform initials is more than the activity of ray initials. So, more secondary vascular tissues are formed than secondary medullary rays. In such condition, secondary medullary rays are compressed by secondary vascular tissues and form vascular rays. Vascular rays are uniseriate (one cell in breadth) or multiseriate (two or more cells in breadth). Vascular rays present in between the secondary xylem are called wood or xylem rays while the part of rays present in between secondary phloem are called phloem rays (see Figure 19).

Formation of annual ring or growth ring

The activity of cambium ring in temperate region is not uniform in spring or rainy season (favourable condition) and in cold winter or dry hot summer (unfavourable condition). Cambium ring is more active during favourable condition and form wide vessels containing more secondary xylem, called spring wood (early wood). But during unfavourable condition, a few small vessels containing secondary xylem, called autumn wood (late wood), are formed. These two kinds of wood appear together as a concentric ring in a year, called annual ring or growth ring. Such rings are formed year after year. Such type of annual ring is called ring porous wood.

The vessels in secondary xylem of the trees of tropical region are almost similar in both the spring wood and autumn wood due to the uniform activity of cambium ring in spring and autumn season. Such type of annual ring is called diffuse porous wood.

Formation of heartwood and sapwood

In old trees, secondary xylem is differentiated into centrally located dark and non-functional wood due to deposition of phytochemicals (oils, resins, gums, tannins, etc.) in cell wall and cell cavities, called heartwood or duramen and the outer light-coloured wood without phytochemicals in cell wall and cell cavities, called sapwood or laburnum (See Figure 20). Heartwood is stronger and more durable than sapwood.

Secondary growth in extra-stellar regions due to activity of cork cambium

The outermost layer of cortical cells (rarely deeper cortical cells/ pericycle cells) goes dedifferentiation and form a meristematic layer of cambium, called cork cambium (phellogen). Phellogen cells divide on both the outer side and inner side to form secondary tissues. The secondary tissue produced on the inner side of phellogen is called secondary cortex or phelloderm (See Figure 21). Phellogen produce cork cells or phellem on the outer side. It consists of dead and compactly arranged rectangular cells that possess suberized cell wall. The activity of cork cambium is more towards outer side than inner side. So, more phellem towards outer side than phelloderm towards inner side are formed. Cork cells prevent the loss of water by evaporation. They also protect the interior cells against the entry of harmful micro-organisms, mechanical injury, and unfavourable conditions.

At places of former stomata of the epidermis, phellogen produces aerating pores in the bark of plant instead of cork cells, called lenticels, for the exchange of gases and transpiration. They occur in woody trees but not in climbers. Lenticels are filled up by loosely arranged thin walled rounded, suberized or unsuberised cells, called complementary cells, which are formed by the division of sub-stomatal parenchyma cells. Complementary cells help in exchange of gases.

Figure 17 Secondary growth in dicot stem: a. & b. promeristem; c. young stem; d. formation of cambium ring; e. secondary growth in the first year; f. secondary growth in third year.

Figure 18 L. S. of Cambium ring showing fusiform and ray initials

Figure 19 T. S. of a part of two years old dicot stem showing secondary growth

Figure 20 T. S. of trunk showing sapwood and heartwood

Figure 21 Secondary growth due to activity of cork cambium