Glossary: Organisation of Plant Tissues and Growth

This chapter's terms in context

Chapter 16 introduces three terms describing how a plant's tissues are organised and how the plant grows: meristem, xylem and phloem. A meristem is a region of a plant made up of continuously dividing, unspecialised cells, and it is the only place in a plant where new cells are produced by mitosis; unlike an animal, whose growth slows and mostly stops after cells differentiate, a plant retains meristems throughout its life and can therefore keep growing indefinitely from these specific growing points. The apical meristem, found at the tips of roots and shoots, produces growth in length (primary growth), while the lateral meristem, found as a cylinder within the stem and root, produces growth in girth (secondary growth) in woody plants, a distinction examiners frequently test by asking a candidate to identify which type of meristem is responsible for a stated direction of growth.

Xylem and phloem are the two types of vascular tissue that run throughout a plant, and although they are often taught and drawn together, they perform opposite and non-interchangeable transport roles. Xylem is composed mainly of dead, hollow, lignified vessel elements joined end to end into continuous tubes, and it transports water and dissolved mineral salts in one direction only, upward, from the roots to the leaves, a movement driven mainly by transpiration pull rather than by any living pumping mechanism inside the xylem itself; because xylem cells are dead at maturity, they cannot actively move substances, and their rigid, lignified walls also provide mechanical support that helps the plant stay upright.

Phloem, in contrast, is composed of living cells, sieve tube elements and companion cells working together, and it transports dissolved food, mainly sucrose produced by photosynthesis, in a process called translocation; phloem transport can move in more than one direction within the same plant at the same time, from a source (such as a mature photosynthesising leaf) to a sink (such as a growing shoot tip, developing fruit, or storage root) wherever that sink currently is, unlike the fixed upward-only direction of xylem. The single most common exam error in this chapter is describing phloem transport as travelling in the same fixed direction as xylem, when its direction actually depends on where sucrose is being produced and where it is currently needed, which can change with the season or the plant's stage of development.

Comparing xylem and phloem side by side, living versus dead cells, direction of transport, the substance transported, and the driving mechanism behind each, is one of the most frequently asked table-style questions built from this chapter, and questions on meristems are frequently combined with this comparison by asking where in the plant new xylem and phloem cells are actually produced, which is the lateral meristem in a plant undergoing secondary growth.

Paper 2 questions on this chapter often supply a cross-section diagram of a stem or root and ask a candidate to identify which vascular tissue is which purely from its position and cell structure, rather than from a label already given, so recognising xylem by its thick, often star-shaped lignified vessels and phloem by its softer, thinner-walled sieve tubes is a skill worth practising directly on unlabelled diagrams rather than only on pre-labelled textbook figures.

A related exam angle asks a candidate to explain why a ringed (girdled) stem, where a strip of bark and the phloem beneath it has been removed all the way round, eventually kills the roots even though water continues to rise through the undamaged xylem: because sugar produced in the leaves can no longer be translocated down past the ring to reach the roots, the roots are gradually starved of food while remaining fully supplied with water, a scenario that tests whether a candidate truly understands that xylem and phloem are independent systems rather than two names for one transport pathway.