Organisation of Plant Tissues and Growth
Plants, like animals, are built from tissues organised for specific jobs. This chapter covers the main plant tissues, how meristematic tissue drives growth, and how a plant's growth is measured with a growth curve.
Knowing which tissue is meristematic and which is permanent is the foundation for the plant transport chapter.
Permanent tissue is further divided into simple and complex types. Simple tissues are made of one cell type: parenchyma cells are thin-walled and carry out storage or, in the leaf mesophyll, photosynthesis; collenchyma cells have unevenly thickened walls that give flexible support to young, growing stems; and sclerenchyma cells are dead at maturity, with thick, lignified walls that give rigid, permanent support.
Complex tissues, xylem and phloem, are each made of more than one cell type working together.
A typical growth curve is S-shaped (sigmoid): growth starts slowly in the lag phase as cells prepare to divide, speeds up sharply in the exponential (log) phase as cell division and elongation peak, then levels off in the stationary phase once the plant approaches its mature size. The rate at any point is affected by external factors such as light, temperature, water and mineral availability, and by internal plant hormones such as auxin, which promotes cell elongation near the shoot and root tips.
In many dicotyledonous stems, a ring of vascular cambium between the xylem and phloem divides to produce secondary xylem and secondary phloem, increasing the stem's girth, this secondary growth is separate from the primary growth in length that happens at the tips. Exam questions on this chapter often give a diagram of a root or stem tip, or a set of growth data to plot and interpret, so practising both skills together pays off.
The two complex tissues differ sharply in whether their cells are alive. A xylem vessel is dead at maturity, a hollow, lignified tube with no cytoplasm, which suits it to carrying water under tension without collapsing.
Phloem, by contrast, is built from living sieve tube elements, which lack a nucleus but are supported and kept functioning by companion cells alongside them, an arrangement suited to actively loading and unloading dissolved sugars.
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Content standards in this chapter
Key concepts
- Plant tissues
- Meristematic tissue divides to make new cells; permanent tissues include epidermis, parenchyma, xylem and phloem.
- Meristematic tissue
- Found at root and shoot tips (apical) and in the cambium; its cells divide by mitosis to make the plant grow.
- Xylem and phloem
- Xylem transports water and mineral salts; phloem transports the products of photosynthesis.
- Primary growth
- Growth in length from the apical meristems at the tips of roots and shoots.
- Zones of growth
- Behind a root tip are the zones of cell division, cell elongation and cell differentiation.
- Growth curve
- Plotting a measure of growth against time gives a curve that shows the rate of growth over the plant's life.
- Simple permanent tissue
- Parenchyma cells are thin-walled and carry out storage or photosynthesis; collenchyma cells have unevenly thickened walls that give flexible support to young stems; and sclerenchyma cells are dead at maturity with thick, lignified walls that give rigid, permanent support.
- Secondary growth
- In many dicotyledonous stems, a ring of vascular cambium lying between the xylem and phloem divides to produce secondary xylem and secondary phloem, increasing the stem's girth over time; this is separate from the primary growth in length that occurs at the apical meristems.
- Phases of the growth curve
- A typical sigmoid growth curve has a lag phase, where growth is slow as cells prepare to divide; an exponential (log) phase, where growth is fastest as cell division and elongation peak; and a stationary phase, where growth levels off as the plant nears its mature size.
- Factors affecting growth
- Plant growth rate is affected by external factors such as light intensity, temperature, water supply and mineral availability, and by internal plant hormones such as auxin, which is produced at the shoot tip and promotes cell elongation just behind it.
- Auxin and tropisms
- Auxin accumulates unevenly when a shoot is lit from one side or a root is placed sideways, causing cells on one side to elongate faster than the other; this differential growth bends the shoot toward light (phototropism) and the root toward gravity (gravitropism).
- Xylem and phloem cell structure
- A xylem vessel is dead at maturity, forming a hollow, lignified tube with no cytoplasm, well suited to carrying water under tension. Phloem sieve tube elements are living but lack a nucleus, and rely on adjacent companion cells to help actively load and unload the sugars being transported.
- Cork cambium and bark
- As a stem undergoes secondary growth and thickens, a second lateral meristem called the cork cambium (phellogen) forms nearer the surface and produces cork cells, which build up as the protective bark and gradually replace the original epidermis.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- Identifying the zones of cell division, elongation and differentiation in a root.
- Comparing meristematic and permanent tissue.
- Interpreting a plant growth curve.
- Naming a simple permanent tissue (parenchyma, collenchyma or sclerenchyma) from a description of its cell wall and function.
- Explaining secondary growth by the vascular cambium and how it differs from primary growth at the apical meristem.
- Identifying the lag, exponential and stationary phases on a plotted growth curve and explaining what limits growth in the stationary phase.
- Linking auxin distribution to the direction a shoot or root bends in phototropism or gravitropism.
Common mistakes
What students write: Saying all plant cells keep dividing.
What earns the mark: Only meristematic tissue divides; permanent tissues have stopped dividing and are specialised.
What students write: Confusing xylem and phloem.
What earns the mark: Xylem carries water and minerals up; phloem carries food (sugars) to where they are needed.
What students write: Placing cell division in the wrong zone.
What earns the mark: Cell division is nearest the tip; then elongation, then differentiation further back.
What students write: Calling the epidermis a dividing tissue.
What earns the mark: The epidermis is a permanent protective tissue, not a meristem.
What students write: Confusing collenchyma and sclerenchyma because both provide support.
What earns the mark: Collenchyma cells are living, with unevenly thickened walls, and support young growing parts flexibly; sclerenchyma cells are dead at maturity, with thick lignified walls, and support mature parts rigidly.
What students write: Thinking secondary growth happens at the tips of roots and shoots.
What earns the mark: Secondary growth happens at the vascular cambium along the length of the stem, increasing girth, while primary growth in length happens at the apical meristems at the tips.
What students write: Saying growth stops completely in the stationary phase of a growth curve.
What earns the mark: Growth slows and levels off, rather than stopping absolutely, in the stationary phase, usually because a limiting factor such as space, light or nutrients caps further increase in size.
What students write: Believing a shoot bends toward light because light 'pulls' it.
What earns the mark: A shoot bends toward light because auxin accumulates on the shaded side, making cells there elongate more than cells on the lit side, curving the shoot toward the light.
What students write: Thinking xylem vessels are living cells like phloem sieve tubes.
What earns the mark: Xylem vessels are dead at maturity, forming hollow lignified tubes; only phloem contains living cells, sieve tube elements supported by companion cells, adapted for actively transporting sugars.
What students write: Confusing the vascular cambium with the cork cambium.
What earns the mark: The vascular cambium lies between the xylem and phloem and produces secondary xylem and phloem; the cork cambium (phellogen) lies nearer the surface and produces the protective cork cells of the bark, both are lateral meristems but with different products.
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Structures in this chapter
Experiments in this chapter
Frequently asked questions
What is the difference between meristematic and permanent tissue?
What are the zones of growth in a root?
What do xylem and phloem transport?
What is the difference between primary and secondary growth?
Why does a growth curve level off in the stationary phase?
How does auxin cause a shoot to bend towards light?
Are xylem and phloem cells alive?
Source:SRC-DSKP-EN, SRC-FORMAT
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