Meristematic Tissues and Growth
Apical meristems at the root and shoot tips produce primary growth, which increases a plant's length. Lateral meristems such as the vascular cambium and cork cambium produce secondary growth, which increases a plant's girth.
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Apical meristem and primary growth
Apical meristem is located at the tip of the root and the tip of the shoot. Cells produced here divide, elongate and differentiate, causing the root and shoot to grow longer.
This lengthwise growth is called primary growth and is responsible for a plant's height and root depth.
Lateral meristem and secondary growth
Lateral meristem includes the vascular cambium, a ring of dividing cells between the xylem and phloem, and the cork cambium near the stem surface. The vascular cambium produces new xylem towards the inside and new phloem towards the outside each growing season, increasing the diameter of the stem or root.
This widening growth is called secondary growth and is typical of woody dicotyledonous plants.
Comparing primary and secondary growth
| Feature | Primary growth | Secondary growth |
|---|---|---|
| Meristem responsible | Apical meristem | Lateral meristem (cambium) |
| Direction of growth | Increases length | Increases girth/diameter |
| Location | Root tip and shoot tip | Vascular cambium, cork cambium |
| Typical plants | All flowering plants | Woody dicotyledonous plants |
How it is examined
You may be asked to name the type of meristem responsible for a given kind of growth, to distinguish primary from secondary growth using a diagram of a stem, or to explain why annual growth rings form as the vascular cambium produces xylem at different rates across seasons.
Meristematic cells versus permanent cells
Cells at a meristem are small, thin-walled and densely packed with cytoplasm, and they keep the ability to divide by mitosis. This is different from the permanent cells around them, which have already undergone cell differentiation to take on a specialised shape and function, such as a xylem vessel or an epidermal cell, and normally do not divide again.
As new cells are pushed away from the meristem, they pass through a zone of elongation, where they enlarge mainly by taking up water, and then a zone of differentiation, where each cell develops the specific structure needed for its final role. This sequence, division, elongation, differentiation, describes how a root or shoot grows longer from its tip backward.
Common misconceptions
Worked exam-style question
Question. A diagram of a root tip shows three regions labelled P, Q and R from the tip inward: P is nearest the root cap, Q is in the middle, and R is furthest from the tip. In region P, cells are small and divide frequently.
In region Q, cells are longer than in P and do not divide. In region R, cells show distinct shapes, including some forming continuous tubes.
(a) Name the region and the process occurring at P, Q and R. (b) Explain why region P must be located behind the root cap rather than at the very tip itself.
(c) State one structural difference you would expect between a cell in region R that becomes a xylem vessel and a cell in region R that becomes an epidermal cell.
Model answer. (a) P is the zone of cell division, where the apical meristem produces new cells by mitosis. Q is the zone of elongation, where cells increase in length mainly by taking up water.
R is the zone of differentiation, where cells develop specialised structures. (b) The root cap must sit in front of the dividing cells to protect the delicate, actively dividing meristem cells from damage as the root pushes through the soil.
(c) A cell becoming a xylem vessel loses its cytoplasm and end walls and develops a thickened, lignified wall to form a hollow tube for water transport, while a cell becoming an epidermal cell keeps its cytoplasm and forms a flattened, closely packed protective cell.
Practice question
A gardener notices that a bamboo stem increases rapidly in height over a few weeks but stays roughly the same thickness throughout its life, while a mango tree trunk becomes visibly thicker every year in addition to growing taller. Suggest an explanation for this difference in terms of the meristems present in each plant.
Exam tip
Key terms
- Meristem, a region of actively dividing, undifferentiated cells responsible for plant growth.
- Cell differentiation, the process by which a cell develops a specialised structure and function.
- Tissue, a group of similar cells performing the same function, formed after differentiation.
- Xylem, the water-conducting tissue produced by both apical and lateral meristems.
- Phloem, the food-conducting tissue produced by the vascular cambium during secondary growth.
Source:SRC-DSKP-EN
Frequently asked questions
What is the difference between primary and secondary growth?
Why do woody plants form growth rings?
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