Plants have some impressive traits that humans don't have. They can reproduce asexually (i.e. clone themselves) and regenerate damaged parts throughout their lives. If a human loses a limb, they definitely can't grow it back!
All of this is possible because of the plant's stem cells or meristem. Stem cells are unspecialised cells that can differentiate (develop) into any new kind of organs' tissue - leaves, stems, roots, flowers; you name it! Once we humans are fully grown, we no longer have this type of stem cell. So, let's find out more about the meristem, its function, the types that exist and some facts about it.
The Definition of the Meristem
The meristem is a type of plant tissue that is made up of undifferentiated cells that are capable of dividing indefinitely. These meristematic cells are essentially stem cells that are clumped together and can develop into all other types of plant tissues and organs.
Learn more about Cell Differentiation and Stem Cells by checking out our articles!
Meristem tissues are regions of plant cells capable of division and growth.
Meristematic cells are undifferentiated cells, found in the meristem tissue, that can divide into similar cells or give rise to differentiated plant tissues and organs.
Stem cells are a type of cell from which all other specialised cells with specific functions are generated from.
Meristematic cells are totipotent stem cells. This is what makes them so unique.
Totipotent cells have the ability to self-renew and can develop into any specialised plant tissue.
Plant meristematic cells can divide and develop throughout the plant's entire lifespan. This enables plants to keep growing and repairing themselves until they die, unlike most animals.
Fig. 1. If you look at meristematic tissue under a microscope, you will probably see some of the cells undergoing mitosis.
A quick recap on cell division! Mitosis produces two identical daughter cells from a singular parent cell.
Most human cell division occurs via mitosis, except for the production of gametes.
- A cell gets ready to divide. The chromosomes in the cell's nucleus replicate, thicken, and coil up.
- The nuclear membrane disintegrates. The chromosomes line up in corresponding pairs in the middle of the cell.
- The pairs are split up and pulled to opposite sides of the cell.
- New nuclei are formed. The cytoplasm is split between the two cells, and they divide.
These new cells can be used for growth or repair.
To learn more about cell division and mitosis, check out our article on Mitosis and the Cell Cycle!
Types of Meristems
Meristem tissues are categorised according to their location in the plant. Apical meristems are found in the root and shoot tips. There are also lateral meristem tissues found in the middle of the plant. Typically, apical meristems support vertical growth, whilst lateral meristems support plant widening.
But it's worth noting that there are a couple of exceptions to this rule, such as grass!
Grass continues to grow even after it has been mowed and the apical meristem in the shoot has been cut off. This is because most of a grass plant's growth comes from its lateral meristem tissues. The blades of grass grow from the middle rather than the top.
Apical Meristems
Both root and shoot meristems are formed during development. The cells in this region elongate and divide to support the growth and development of new meristematic cells. These new cells will then differentiate into the required specialised cells.
Fig. 2: Apical meristems are found in the tip of shoots and roots, unsplash.com
Radical Meristem
The radical meristem is a small region of stem cells at the tip of a root. These cells can divide and form new root tissue. An outer part of parenchyma cells, known as the root cap, protects the apical meristem inside the soil from external environmental stressors.
The radical meristem allows roots to grow longer, maximising access to water, nutrients, and minerals in the soil.
Shoot Meristem
Shoot meristems (also known as terminal buds) are found at the ends of all branches and stems. These areas can grow and develop indefinitely, possessing the ability to transform into leaves, branches, flowers and fruits. Shoot meristem tissue aids with reproduction and maximises access to sunlight for photosynthesis.
Lateral Meristems
Some meristem tissues are found in the middle of the plant, rather than at the ends.
An example is the cambium. This lateral meristem tissue is located between the xylem and the phloem in a plant stem. It allows the stem to grow wider and form woody tissue, strengthening the plant.
The Function of the Meristems
Meristematic tissue supports multiple functions within the plant.
Primary Growth
Cell elongation and division trigger primary growth.
Primary growth is the increase in a plant's height or root length.
How does this benefit the plant?
Increased height: the plant gets taller. This enables better access to sunlight for photosynthesis.
Increased root length: the plant has a greater ability to access water, minerals and nutrients in the soil.
Wound Repair
Meristem cells can form inside injured tissues, differentiating into new, undamaged cells to control wound healing.
Hormone Production
Three different types of plant hormones control the elongation and division of meristem cells. The hormones are summarised in this table.
| Hormone | Function |
| Gibberellin | Promotes stem elongation |
| Cytokinin | Promotes cell division |
| Auxin | Promotes cell elongation |
Table 1: The hormones and their function in plant growth and development.
Gibberellin is not produced in the meristematic tissue, but cytokinin and auxin are.
Shoot and root tips are the production site of auxin, a significant plant hormone. Auxin stimulates cell elongation. This commonly happens in the 'darker' side of the plant stem that faces away from a light source. Longer cells in the darker side of the stem allow the plant to bend towards the light. Furthermore, auxin controls apical dominance. If the apex of the shoot (and, thus, the site of the apical meristem) is removed, auxin is no longer produced. This triggers the lateral meristems to start growing.
Cytokinins are another vital plant hormone produced in apical meristems. They promote cell division.
Producing Identical Plants
Meristem tissues can differentiate into any type of plant cell. As a result, these cells can be used to produce clones of entire plants. This method is a quick and cheap way to prevent the extinction of rare species. Additionally, this cloning method can be used to grow crops of identical plants with positive traits such as disease resistance or a high yield.
Meristem Example: Uses
Meristematic cells can be used to clone plants quickly and economically. How does it work?
Firstly, the meristematic cells are removed from a plant and grown in tissue culture.
Tissue culture is the growth of cells outside an organism.
The tissue culture is grown in a culture medium, which contains:
Agar: a jelly-like material derived from seaweed, used for support
Nutrients: for growth and repair
Plant hormones: to stimulate growth and cell division
It's much quicker to produce new plants by cloning than by collecting and sowing seeds.
Clones will be genetically identical to the original plant that provided the meristem cells. This is useful for the large-scale cultivation of crops with desirable traits, such as drought tolerance.
Additionally, cloning plants in tissue culture is an easy and effective way of producing new individuals, helping to preserve endangered plant species.
Meristem Tissue: Some Facts
Here are some facts about meristem tissue:
Cloning plants using their meristem tissue is a common propagation method. Meristem tissues have been used to clone potatoes, corn, sugarcane, and bananas.
The oldest tree in the world is a bristlecone pine in California. The Methuselah tree is named after a Biblical figure who lived for almost a millennium. The Methuselah Tree is nearly 5000 years old. In comparison, the world's oldest vertebrate is the Greenland Shark. They live for an average of 400 years.
Meristem - Key takeaways
Meristem tissues are regions of plant cells capable of division and growth. These cells are totipotent - meaning they can differentiate into any specialised cell type.
Apical meristems are found in a plant's shoots and roots, whilst intercalary and lateral meristems are located in the centre of the plant.
The hormones gibberellin and auxin control cell elongation, while cytokinins promote cell division.
The meristem tissue is used for primary growth, wound repair, hormone production and plant cloning.
References
- CGP, GCSE AQA Combined Science Revision Guide, 2021
- Edexcel, Salters-Nuffield A level Biology, 2015
- Kara Rogers, Mitosis, Britannica, 2020
- M.J. Farabee, Plant Hormones, Nutrition, and Transport, 2007
- Melissa Petruzzello, Meristem, Britannica, 2020
- Oregon State University, Mechanisms for Growth, 2022
- Pratima Bajpai, Biermann’s Handbook of Pulp and Paper (Third Edition), 2018
- Reinhard Renneberg, Biotechnology for Beginners (Second Edition), 2017
- Robin McKie, The Methuselah tree and the secrets of Earth’s oldest organisms, The Guardian, 2020
- Srishti Aggarwal, Plant stem cells and their applications: special emphasis on their marketed products, Biotech, 2020
- Susan T. Rouse, Meristems, Encyclopedia.com, 2022
- Thomas Greb, Plant Stem Cells, Current Biology, 2016
- Fig. 1. Mitotic Stages in Apical Meristem of Allium Root Tip 400x (https://www.flickr.com/photos/146824358@N03/23581050748/in/album-72157684892968976/) por Berkshire Community College Bioscience Image Library, Dominio público.
- Fig. 2. Image (https://unsplash.com/photos/ZgY-zd7rH9g) by Gaudenis G. (evergreensanddandelions), free use under the Unsplash License (https://unsplash.com/license)
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