Organisation of Plant Tissues and Growth, revision notes

Overview

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.

Meristematic and permanent tissues (CS 16.1)

Content Standard 16.1 sorts the plant body into two broad tissue types. Meristematic tissue is made of small, thin-walled cells with dense cytoplasm and no large vacuole; these cells keep dividing by mitosis and are the source of every new cell.

Permanent tissue is made of cells that have stopped dividing and become specialised for a particular job.

Meristematic tissue is grouped by position: apical meristems at the tips of roots and shoots drive growth in length, while lateral meristems such as the vascular cambium run along the sides of the stem and root and add to its girth. Permanent tissue is grouped by make-up: simple tissues (parenchyma, collenchyma and sclerenchyma) contain one cell type, while complex tissues (xylem and phloem) contain more than one.

Sorting a named tissue into meristematic or permanent, and then into simple or complex, is the first skill the chapter rewards, because later questions assume you can place any tissue in this framework before describing it.

Simple permanent tissues: parenchyma, collenchyma, sclerenchyma (CS 16.1)

The three simple permanent tissues are told apart by their cell walls and by whether the cells are alive. Parenchyma cells are living, thin-walled and loosely packed; they store food and, in the leaf mesophyll, carry out photosynthesis.

Collenchyma cells are living, with cellulose walls that are unevenly thickened at the corners; this gives flexible support to young, growing stems and leaf stalks without stopping them bending. Sclerenchyma cells are dead at maturity, with thick walls hardened by lignin; they give rigid, permanent support and occur as fibres and sclereids.

Because collenchyma and sclerenchyma both give support, questions often ask you to tell them apart. The reliable test is whether the cells are alive and how the wall is thickened: living with uneven cellulose thickening points to collenchyma, dead with even lignified thickening points to sclerenchyma.

Complex tissues: xylem and phloem (CS 16.1)

The two complex tissues form the plant's vascular system. Xylem carries water and dissolved mineral salts from the roots upward to the leaves; it is built from vessels and tracheids, which are dead at maturity, hollow and lignified, so they form continuous tubes that carry water under tension without collapsing and also help support the plant.

Phloem carries the sugars made in photosynthesis from the leaves to the rest of the plant, a movement called translocation; it is built from living sieve tube elements, which lose their nucleus, together with companion cells that keep them working and help load and unload the sugars.

The sharpest difference is whether the conducting cells are alive. Xylem conducts through dead cells, while phloem conducts through living ones.

Stating this, and linking it to the direction of transport, answers a large share of the questions set on this content standard.

Meristematic tissue and the zones of growth in a root (CS 16.2)

Content Standard 16.2 looks at how meristematic tissue produces growth. Just behind the tip of a root lie three zones in a fixed order.

At the very tip a root cap protects the delicate cells as the root pushes through the soil. Behind it is the zone of cell division, where the apical meristem divides by mitosis to make new cells.

Next is the zone of cell elongation, where those new cells take in water and grow longer, lengthening the root. Furthest from the tip is the zone of cell differentiation, where cells become specialised, for example into root hair cells or xylem.

A common error is to place cell division in the wrong zone, so fix the order from the tip outward: protection, division, elongation, differentiation. Drawing and labelling a root tip with these zones is one of the most frequently set diagram questions in this chapter.

Primary and secondary growth (CS 16.2)

Growth in a plant takes two forms. Primary growth is an increase in length produced by the apical meristems at the tips of roots and shoots.

Secondary growth is an increase in girth produced by a lateral meristem, the vascular cambium, a ring of dividing cells between the xylem and phloem in dicotyledonous stems; it adds secondary xylem on the inside and secondary phloem on the outside, thickening the stem over the growing season. As the stem thickens, a second lateral meristem, the cork cambium, forms nearer the surface and produces cork, which builds up as protective bark.

Keeping the two apart is worth a mark on its own: primary growth happens at the tips and lengthens the plant, while secondary growth happens along the sides at the cambium and thickens it.

Growth curves: absolute growth and growth rate (CS 16.3)

Content Standard 16.3 covers how growth is measured and plotted. Plotting a measure of size or dry mass against time gives an absolute growth curve, which for a whole plant is sigmoid (S-shaped).

It 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.

Plotting the rate of growth, the change in size per unit time, against time instead gives a bell-shaped curve that rises to a peak during the exponential phase and falls back to near zero in the stationary phase. Dry mass is the more reliable measure because it excludes the changing water content of the plant, but measuring it kills the sample, so a set of similar plants must be sampled at intervals rather than one plant measured over and over.

A growth curve levels off in the stationary phase not because growth stops entirely but because a limiting factor, such as space, light, water or a mineral ion, caps further cell division and elongation. Reading a plotted curve, naming its phases and explaining what limits it are the three skills this content standard is examined on.

Auxin, tropisms and factors affecting growth

The rate of plant growth depends on external factors, including light intensity, temperature, water supply and the availability of mineral ions, and on internal plant hormones, chiefly auxin. Auxin is made at the shoot tip and promotes the elongation of cells just behind the tip.

Auxin also explains tropisms, the directional growth responses of plants. When light comes from one side, auxin moves to and builds up on the shaded side of the shoot; the cells there elongate more, so the shoot curves toward the light, a response called positive phototropism.

In a root placed on its side, auxin builds up on the lower side and slows elongation there, so the root curves downward toward gravity, a response called positive gravitropism. The same hormone therefore produces opposite bends in shoots and roots because the two organs respond to it differently.

Key concepts to master

• 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.

Quick recall checklist

1. Can you define and explain Plant tissues?
2. Can you define and explain Meristematic tissue?
3. Can you define and explain Xylem and phloem?
4. Can you define and explain Primary growth?
5. Can you define and explain Zones of growth?
6. Can you define and explain Growth curve?
7. Can you define and explain Simple permanent tissue?
8. Can you define and explain Secondary growth?
9. Can you define and explain Phases of the growth curve?
10. Can you define and explain Factors affecting growth?
11. Can you define and explain Auxin and tropisms?
12. Can you define and explain Xylem and phloem cell structure?
13. Can you define and explain Cork cambium and bark?

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Source:SRC-DSKP-EN