Difference Between Primary and Secondary Growth

The key difference between primary and secondary growth is that primary growth increases the length of roots and shoots as a result of cell division in the primary meristem while secondary growth increases the thickness or the girth of the plant as a result of cell division in the secondary meristem.

Primary and secondary growth allow plants to increase in size – length and thickness. Apical and lateral meristems are responsible for plant growth. When the cells of the apical meristem divide, primary growth occurs. In contrast, when the cells of the lateral meristem divide, secondary growth occurs. Primary growth is responsible for the increase in the length of the shoot while secondary growth is responsible for the increase of the girth of the plant.

CONTENTS

1. Overview and Key Difference
2. What is Primary Growth
3. What is Secondary Growth
4. Similarities Between Primary and Secondary Growth
5. Side by Side Comparison – Primary vs Secondary Growth in Tabular Form
6. Summary

What is Primary Growth?

Primary growth of plants is the process of increasing the length of the shoots and roots. It occurs as a result of the cell division in primary meristems such as apical meristem, intercalary meristem, and intrafascicular cambium. Shoot apex is dome-shaped with leaf primordia. There are axillary buds, nodes, and internodes. Moreover, the apex has three distinct regions. At the very top is the region of cell division where only cell division takes place. Next to that, there is a region of cell enlargement. Behind this region is the region of cell differentiation where each cell becomes fully specialized for its particular function.

Furthermore, three types of basic meristematic tissues occur at the stem apex. They are the protoderm, procambium, and ground meristem. The procambium is a series of longitudinally running strands. In a cross-section, they appear in the form of a broken ring. Procambium produces primary vascular tissues. The first formed cells are protoxylem to the inside and protophloem to the outside. Moreover, the protoxylem typically has only annular and spiral thickenings of lignin, allowing elongation to take place. Other thickenings occur only after elongation. Furthermore, the cavities of the protoxylem are much smaller. Soon protoxylem and protophloem become inactive. Their function is taken over later by developing metaxylem and metaphloem.

What is Secondary Growth?

After primary growth, lateral meristem becomes active and results in the formation of secondary permanent tissues. It is called secondary growth. The lateral meristems are the lateral vascular cambium and cork cambium. They are formed only on dicots. In monocots, there is no cambium. Therefore, there is no secondary growth. As a result of secondary growth, there is an increase in thickness or girth in stems and roots. In the stem, the intrafascicular cambium becomes active and cut off cells to the outside and inside. The cells that cut off to the outside become the secondary phloem while the cells on the inside become the secondary xylem.

In the meantime, parenchyma cells between adjacent vascular bundles also become meristematic and form the interfascicular cambium. The intrafascicular cambium and the interfascicular cambium join to form a cambial ring, which is the vascular cambium. The interfascicular cambium cuts off cells to the outside and inside. The outside cells become the secondary phloem while inside cells become the secondary xylem. The cambium contains fusiform initials and ray initials. Fusiform initials give rise to normal xylem and phloem. Ray initials give rise to parenchyma, which forms medullary rays.

As the numbers of cell layers inside increase, the cells outside become compressed and this results in the formation of another lateral meristem in the outer layers of the cortex. These become a ring of cork cambium. Cork cambium cuts off cells to the inside and outside. Cells that are cut off to the outside become suberized and form the cork. Cells that cut off to the inside form secondary cortex.

What are the Similarities Between Primary and Secondary Growth?

• Primary and secondary growth happen in plants, and they allow plants to increase in size permanently.
• Furthermore, primary and secondary growth occur as a result of the rapid cell division in the meristematic tissues.
• Additionally, in woody plants, primary growth is followed by secondary growth.

What is the Difference Between Primary and Secondary Growth?

Primary growth is the process that increases the length of the plant while secondary growth is the process that increases the girth of the plant. Thus, this is the key difference between primary and secondary growth. A further difference between primary and secondary growth is that the primary growth is a result of cell division in the primary meristems while secondary growth is a result of the cell division in the secondary meristems.

Below infographic shows more details on the difference between primary and secondary growth.

Summary – Primary vs Secondary Growth

Plants grow in two ways: primary growth and secondary growth. Primary growth is the increase in the length of the plant. In contrast, secondary growth is the increase in the girth of the plant. Moreover, meristematic tissues, which contain undifferentiated cells, are responsible for primary and secondary growth. Primary growth occurs as a result of the cell division in the primary meristems, mainly in the apical meristems located at root and shoot tips, while secondary growth occurs as a result of the cell division in the secondary meristems such as cork cambium and vascular cambium of woody plants. Thus, this summarizes the difference between primary and secondary growth.

Reference:

1. “30.2C: Primary and Secondary Growth in Stems.” Biology LibreTexts, Libretexts, 16 July 2018, Available here.
2. “Plant Development II: Primary and Secondary Growth.” Biology 1520, 19 Feb. 2018, Available here.

Image Courtesy:

1. “Apical Meristem in Allium Root Tip” By Berkshire Community College Bioscience Image Library (Public Domain) via Flickr
2. “Tree secondary growth diagram” By Chiswick Chap – Own work (CC BY-SA 4.0) via Commons Wikimedia