Difference Between Primary and Secondary Growth in Plants

Main Difference – Primary Growth vs Secondary Growth

Primary growth and secondary growth are the two mechanisms by which woody plants increase its size. Both primary and secondary growth of a plant can be either determinate or indeterminate. The determinate growth stops when the elements of the plant such as leaves reach a particular size, while the indeterminate growth occurs continuously throughout the lifetime of the plant. The main difference between primary and secondary growth is that primary growth increases the length of the plant whereas secondary growth increases the thickness. The herbaceous plants only undergo primary growth. But, woody plants undergo both primary and secondary growth. The apical meristem causes the primary growth whereas the lateral meristem causes the secondary growth of the plant.   

Key Areas Covered

1. What is Primary Growth
      – Definition, Characteristics, Role
2. What is Secondary Growth
      – Definition, Characteristics, Role
3. What are the Similarities Between Primary and Secondary Growth
      – Outline Of Common Features
4. What is the Difference Between Primary and Secondary Growth
      – Comparison of Key Differences

Key Terms: Apical Meristem, Cork Cambium, Epidermis, Lateral Meristem, Periderm, Phloem, Primary Growth, Secondary Growth, Vascular Cambium, Woody Plants, Xylem

What is Primary Growth

The primary growth is the increase in the length of both shoot and root of a plant. It occurs due to continuous cell divisions in the apical meristem. This means the primary growth occurs in the apices of shoots and roots. Immediately after the cell division, the cells are elongated and differentiated to the relevant tissues. The cells produced by the apical meristem are called the primary meristem. The primary meristem is differentiated into the protoderm, ground meristem, and procambium, which give rise to the epidermis, ground tissue, and vascular bundles respectively.

Figure 1: Dicot Stem and Monocot Stem

The axillary shoots are developed by the parallel mitotic cell divisions in the cortex. The apical dominance refers to the diminishing of the growth of the axillary buds by the apical meristem. The cut off of the apical meristem gives a bushy shape to the plant. The increase of the length of the shoot allows the plant to absorb sunlight efficiently. Moreover, the increase in the depth of the root allows the plant to absorb water efficiently. The primary stem of monocots and dicots are shown in figure 1. 

What is Secondary Growth

The increase of the thickness of the shoot and the root of the plant is referred to as the secondary growth. The secondary growth occurs due to the action of the lateral meristem. It only occurs in woody plants. The herbaceous plants do not undergo secondary growth. The lateral meristem is composed of the vascular cambium and the cork cambium. The vascular cambium is located in between the primary xylem and primary phloem. The cells of the vascular cambium that are next to the primary phloem divide to form the secondary phloem. The secondary phloem consists of sieve elements and companion cells. The cells near to the primary xylem forms the secondary xylem, which consists of tracheids and vessels. The activity of the vascular cambium during the summer and the winter give rise to annual growth rings.

Figure 2: Mature Pinus Ring
A – Pith, B – Secondary xylem, C – Primary xylem, D – Resin duct, E – Cortex, F- Vascular cambium, G – Epidermis

The tough, outermost layer or the bark is formed by the cork cambium. The cells in the bark contain a waxy substance called suberin, which reduces the water loss. The phelloderm layer of the bark grows towards the stele. The phelloderm, cork cambium, and the cork cells are collectively called the periderm. In mature plants, the epidermis is replaced by the periderm. The lenticels are the openings of a mature stem through which the gas exchange occurs. A mature Pinus stem is shown in figure 2. 

Similarities Between Primary and Secondary Growth

• Primary and secondary growth are involved in increasing the size of the plant.
• Both primary growth and secondary growth occur in woody plants.
• The meristematic tissue is involved in both primary and secondary growth of the plant.

Difference Between Primary and Secondary Growth

Definition

Primary Growth: Primary growth is the growth that occurs by the action of the primary meristem, which increases the length of the stem and adds appendages to the stem.

Secondary Growth: Secondary growth is the growth that occurs by the action of the cambium, which increases the diameter of the plant.

Occur by

Primary Growth: The primary growth occurs by the action of the apical meristem.

Secondary Growth: The secondary growth occurs by the action of the lateral meristem.

Growth

Primary Growth: The primary growth results in the growth in the longitudinal axis.

Secondary Growth: The secondary growth results in the radial growth.

Order

Primary Growth: The primary growth of the plant occurs at the beginning.

Secondary Growth: The secondary growth follows the primary growth.

Duration

Primary Growth: The primary growth stops after completion of the tissue differentiation. 

Secondary Growth: The secondary growth only occurs in the mature parts (parts that are completely developed).

Location

Primary Growth: The primary growth occurs in all parts of all plants.

Secondary Growth: The secondary growth occurs in both angiosperms and gymnosperms (except monocots)

Development

Primary Growth: The epidermis, cortex, and the primary vascular tissues are developed during the primary growth.

Secondary Growth: The bark, periderm, lenticels, secondary phloem, and secondary xylem are developed during the secondary growth.

Conclusion

Primary growth and secondary growth are the two types of mechanisms that plants use to increase the size. The primary growth occurs by the action of the apical meristem while the secondary growth occurs by the action of the cambium. The primary growth increases the length of the plant while the secondary growth increases the diameter of the plant. The main difference between primary and secondary growth is the type of tissues involved in each growth and the type of growth.

Reference:

1.“ Primary growth.” Afzender, Available here. Accessed 27 Aug. 2017.
2.”Secondary Growth.” Botany 103, University of Wisconsin. Available here. Accessed 27 Aug. 2017.

Image Courtesy:

1. “Figure 30 02 06” By CNX OpenStax – Available here, (CC BY 4.0) via Commons Wikimedia
2. “Pinus mature stem L” By Jon Houseman – Jon Houseman and Matthew Ford (CC BY-SA 4.0) via Commons Wikimedia