Form 5 · Worked answers

Organisation of Plant Tissues and Growth, worked answers

Fully worked answers for Organisation of Plant Tissues and Growth, original structured and essay questions with mark-scheme keywords highlighted.

Book a Trial Class

One-hour paid trial · Same-day reply

How this topic is examined

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.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

State three differences between meristematic tissue and permanent tissue.

[4]

Show answer

Meristematic tissue is made of small, thin-walled cells that keep dividing by mitosis, while permanent tissue is made of cells that have stopped dividing. Meristematic cells have dense cytoplasm and no large vacuole, while permanent cells are often larger and may have a large vacuole or thick walls. Meristematic tissue is found at the root and shoot tips and in the cambium, while permanent tissue makes up the bulk of the plant body. Meristematic tissue makes the plant grow, while permanent tissue carries out specialised functions such as transport or support.

divides by mitosisstopped dividingthin-walledspecialisedtips and cambium

A simple permanent tissue has cells that are dead at maturity with thick, lignified walls. (a) Name the tissue. (b) State its function. (c) State one way it differs from collenchyma.

[4]

Show answer

(a) The tissue is sclerenchyma. (b) It gives rigid, permanent support to the mature parts of the plant. (c) Sclerenchyma cells are dead at maturity with evenly thickened, lignified walls, whereas collenchyma cells are living with unevenly thickened cellulose walls that give flexible support to young, growing parts.

sclerenchymadead at maturitylignifiedrigid supportcollenchyma living

Compare the structure and function of xylem and phloem.

[6]

Show answer

Xylem transports water and dissolved mineral salts from the roots upward to the leaves, while phloem transports the sugars made in photosynthesis from the leaves to the rest of the plant. Xylem is built from vessels and tracheids that are dead at maturity, hollow and lignified, so they form continuous tubes that carry water without collapsing and also help support the plant. Phloem is built from living sieve tube elements, which lose their nucleus, supported by companion cells that help load and unload the sugars. The clearest difference is that xylem conducts through dead cells while phloem conducts through living cells.

water and mineralssugarsdead lignified vesselsliving sieve tubescompanion cellstranslocation

Describe the three zones of growth found just behind the tip of a root.

[4]

Show answer

Behind the root cap, which protects the tip, is the zone of cell division, where the apical meristem divides by mitosis to produce new cells. Next is the zone of cell elongation, where the new cells take in water and grow longer, pushing the root through the soil and increasing its length. Furthest from the tip is the zone of cell differentiation, where the cells become specialised, for example into root hair cells for absorption or into xylem for transport.

zone of cell divisionapical meristemzone of elongationzone of differentiationroot hair cells

Explain the difference between primary growth and secondary growth in a plant.

[4]

Show answer

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 (thickness) produced by the vascular cambium, a ring of meristematic tissue between the xylem and phloem in dicotyledonous stems. The cambium divides to add secondary xylem and secondary phloem, widening the stem, while primary growth at the tips makes the plant taller and its roots longer. Primary growth happens at the tips; secondary growth happens along the sides.

increase in lengthapical meristemincrease in girthvascular cambiumsecondary xylem and phloem

A plant's growth is plotted as size against time, giving a sigmoid curve. (a) Name the three phases of the curve. (b) Explain why the curve levels off in the last phase.

[5]

Show answer

(a) The three phases are the lag phase, the exponential (log) phase and the stationary phase. In the lag phase growth is slow as cells prepare to divide; in the exponential phase growth is fastest as cell division and elongation peak; in the stationary phase growth levels off. (b) The curve levels off in the stationary phase because a limiting factor, such as space, light, water or a mineral ion, caps further cell division and elongation. Growth does not stop entirely, but the rate of new growth falls to near zero as the plant reaches its mature size.

lag phaseexponential phasestationary phaselimiting factorlevels offmature size

Explain, in terms of auxin, how a shoot bends towards light coming from one side.

[5]

Show answer

Auxin is made at the shoot tip and promotes the elongation of cells just behind the tip. When light comes from one side, auxin moves to and builds up on the shaded side of the shoot. Because auxin promotes elongation, the cells on the shaded side elongate more than those on the lit side. This unequal growth makes the shoot curve towards the light, a response called positive phototropism. The bend is therefore caused by uneven distribution of auxin and unequal cell elongation, not by the light pulling the shoot.

auxinshoot tipshaded sideelongationunequal growthpositive phototropism

Phrasing that earns marks

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.

Frequently asked questions

What is the difference between meristematic and permanent tissue?

Meristematic tissue is made of small, thin-walled cells that keep dividing by mitosis to produce new cells, and it is found at the tips of roots and shoots and in the cambium. Permanent tissue is made of cells that have stopped dividing and become specialised for a job, such as the epidermis for protection or xylem for transport. Meristematic tissue makes the plant grow; permanent tissue carries out functions.

What are the zones of growth in a root?

Just behind the root tip there are three zones. In the zone of cell division, meristematic cells divide to make new cells. In the zone of cell elongation, the new cells grow longer, pushing the root through the soil. In the zone of cell differentiation, the cells become specialised, such as into root hair cells or xylem.

What do xylem and phloem transport?

Xylem transports water and dissolved mineral salts from the roots upward to the leaves. Phloem transports the sugars made in photosynthesis from the leaves to other parts of the plant that need energy or storage. The two tissues together form the plant's vascular system.