Organisation of Plant Tissues and Growth, practice questions

B, Meristematic tissue is made of dividing cells; the others are permanent tissues.

C, Sclerenchyma cells are dead at maturity with thick, lignified walls that give rigid support.

B, Phloem transports sugars from the leaves to the rest of the plant, a process called translocation.

C, The zone of cell division, containing the apical meristem, lies just behind the protective root cap.

C, The vascular cambium, a lateral meristem between xylem and phloem, produces secondary growth in girth.

B, Xylem vessels and tracheids are dead at maturity, hollow and lignified.

B, Growth is fastest in the exponential phase, when cell division and elongation peak.

B, A limiting factor such as space, light or nutrients caps further division and elongation.

B, Auxin builds up on the shaded side, making those cells elongate more, so the shoot curves towards the light.

C, Dry mass excludes the changing water content of the plant, making it more reliable than wet mass.

• (a) A simple permanent tissue contains only one type of cell, while a complex permanent tissue contains more than one type of cell working together.
• (b) The three simple permanent tissues are parenchyma, collenchyma and sclerenchyma.
• (c) Parenchyma has thin cellulose walls and carries out storage and, in the leaf, photosynthesis.
• Collenchyma has unevenly thickened cellulose walls and gives flexible support to young growing parts.
• Sclerenchyma has thick, lignified walls and gives rigid, permanent support.
• The complex tissues xylem and phloem together form the vascular system.

• (a) The root cap protects the delicate cells of the root tip as it pushes through the soil.
• (b) The zone of cell division, where the apical meristem divides to make new cells.
• The zone of cell elongation, where the new cells take in water and grow longer.
• The zone of cell differentiation, where the cells become specialised, for example into root hair cells.
• (c) Cells in the zone of cell division divide by mitosis.
• Primary growth at the tip therefore increases the length of the root.

• (a) Dry mass is used because it excludes the changing water content of the plant, making it a more reliable measure of growth.
• Measuring dry mass kills the sample, so a set of similar plants is sampled at intervals.
• (b) The three phases are the lag phase, the exponential (log) phase and the stationary phase.
• (c) In the stationary phase a limiting factor caps further growth.
• The limiting factor may be space, light, water or the supply of a mineral ion.
• Growth slows and levels off rather than stopping completely, as the plant nears its mature size.

Meristematic tissue divides to make new cells; permanent tissues include epidermis, parenchyma, xylem and phloem.

Found at root and shoot tips (apical) and in the cambium; its cells divide by mitosis to make the plant grow.

Xylem transports water and mineral salts; phloem transports the products of photosynthesis.

Growth in length from the apical meristems at the tips of roots and shoots.

Behind a root tip are the zones of cell division, cell elongation and cell differentiation.

Plotting a measure of growth against time gives a curve that shows the rate of growth over the plant's life.

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.

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.

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.

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 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).

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.

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.