Stem is the plant axis that bears buds and shoots with leaves and, at its basal end, roots. It is composed of three tissue systems that include the epidermis, vascular, and ground tissues, all of which are made from the simple cell types.
In this article, the Primary Structure of Stem in Monocot and Dicot plants will be discussed explicitly.
Primary Structure of Dicot Stem
In Dicot plants, Stem consists of Epidermis, Endodermis, Cortex, Parenchyma, Collenchyma, Sclerenchyma, Stele, Pericycle, Vascular bundles etc.
These are described below:
Epidermis is the outermost layer of the stem and consists of single layer of cells.
- Outer wall of epidermis is thickened and heavily cutinized. Do not possess intercellular space.
- Contain stomata and produce various types of trichomes.
- Cells are compactly arranged.
- In TS cells are rectangular.
- It restricts the rate of transpiration.
- It protects the underlying tissue from mechanical injury and from disease
Next to the epidermis cortex present.
- Innermost layer of cortex is called endodermis.
- The part of the cortex situated between the epidermis and endodermis is generally divided into two regions:
- Outer zone of collenchyma cell.
- Inner zone of parenchyma cell.
- Strengthening material in succulent organs (where no woody tissue) and in the soft young part of woody plants.
- The collenchyma cell of stems sometimes contain chloroplast, thereby carry on photosynthesis.
On the inside of the epidermis, there is usually band of collenchyma.
- The cells of the collenchyma are modified parenchyma cells with cellulose walls thickened at the angles.
- Collenchyma cell of stem sometimes contain chloroplast.
- Strengthening material in succulent organ.
- Collenchyma cells carry on photosynthesis.
Parenchyma cells are regular in shape, thin walled, living cell not elongated, develop chloroplast (chlorenchyma cell) when exposed to light.
- Slow conduction of water.
- Sclerenchyma cells are found in the cortex of some plants (e.g. Nymphea), water plant.
- Stone cells are short & irregular shape and sclerenchyma fiber (thick walled & dead cell).
- To provide strength.
Endodermis is the innermost layer of cortex.
- Barrel shaped cell.
- Elongated, compact cells. No intercellular space among the cells.
- It surround stele.
Stele is a part of the plant stem inside the cortex. It has three regions:
- Vascular bundle
These three regions are described below:
- It occurs between the vascular bundle & the cortex.
- Generally composed of parenchyma and sclerenchyma cells but sclerenchyma
may be absent.
2. Vascular Bundle
- In cross section, it arranged in broken ring. Consists of three parts:
• Centre of the stem, thick walled cells.
• Peripheral portion of VB and composed of thin walled cell.
• Meristematic cell which separates Xylem and Phloem.
Xylem formed first is nearest the center of the stem.
Peripheral part of the primary xylem.
Centre of the stem, composed of thin walled parenchyma cells. Intercellular spaces are present.
Primary Structure of Monocot Stem
Monocot stems are similar to dicotyledonous stem in having an epidermis , cortex and a stele.
Cortex may be well developed and sharply marked off from the stele or it may be quite narrow and inconspicuous.
• Vascular bundle of monocot stems are usually scattered throughout the stele including the pith so that there is no distinction between pith and pith ray. Sometimes the center of the stele is free form VB and is occupied by parenchyma cells which dry up and disappear at an early stage resulting in a hollow stem.
- The VB of monocot stems are like those of dicot stem in consisting of xylem towards the center of the stele and phloem towards the periphery. The VB of monocot stem do not possess a cambium layer which is found in dicot stem. This means that monocotyledon stems usually do not have secondary
- Each bundle remains more or less completely surrounded by a sheath of sclerenchyma cells, the bundle sheath which is particularly well developed on the sides towards the center and towards the periphery of the stem.
- The phloem is made up of mostly of sieve tube, companion cells and xylem – vessel and woody parenchyma.
Characteristics of Monocot Stem
The most distinctive and characteristic features of the monocot stem are as
- The Vascular bundle are many.
- The stele is broken up into bundles. The VB are lying scattered in the ground tissue of the axis.
- The endodermis is not found. The cortex, pericycle and pith are not differentiated because of the presence of scattered bundles throughout the axis.
- The VB are collateral and closed. The secondary growth of monocot is
lacking but vestiges of cambial in bundles may present in the plant.
1. Class Lecture of Parveen Rashid, PhD
Professor, Department of Botany, University of Dhaka.