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Primary and secondary growth is taught in the note for Growth in Plants. Read it below and look for the section on this topic.

Growth in Plants

14 min read

The cell cycle

Growth needs cells to divide. The cell cycle has interphase (G1, S and G2) and mitosis (M phase). In S phase the DNA is copied, so each daughter cell gets a full set of chromosomes.

The cell cycleA circular diagram of the life of a dividing cell. Interphase takes most of the time and has three parts: G1, when the cell grows, S, when the DNA is copied, and G2, when the cell prepares to divide. Mitosis follows, divided into prophase, metaphase, anaphase and telophase, and then the cell splits in cytokinesis.cellcycleG1: cell growsS: DNA copiedG2: preparesM: mitosisthen cytokinesis
Stages of mitosisFive drawings. In interphase the DNA is copied. In prophase the chromosomes become visible. In metaphase they line up across the centre. In anaphase the two halves of each chromosome are pulled to opposite ends. In telophase and cytokinesis two new nuclei form and the cell divides into two identical daughter cells.InterphaseDNA copiedProphasechromosomes appearMetaphaseline up at the equatorAnaphasechromatids pulled apartTelophase and cytokinesistwo identical cells

Growth patterns

The growth of a whole plant follows an S-shaped curve. Annual plants (such as maize) grow in a single season and die. Perennials grow for many years. Growth is measured by increase in height, fresh mass or dry mass.

A sigmoid growth curveA graph of size against time. Growth starts slowly in the lag phase, speeds up in the exponential or log phase when resources are plentiful, slows as resources run out, and levels off in the stationary phase. The whole curve is S-shaped. Populations of bacteria and many organisms follow the same pattern.timesize or numberlag phaselog (exponential)stationary phasecarrying capacity

Seed dormancy and viability

A dormant seed does not germinate even when conditions are suitable. Dormancy is caused by a hard seed coat, a growth inhibitor such as abscisic acid, or an immature embryo. It is ended by soaking, scratching the coat (scarification), cold treatment or gibberellin. Viability is how long a seed stays alive and able to germinate: a tetrazolium test colours living seeds red.

A seed and germinationA bean seed has a seed coat, two seed leaves called cotyledons that store food, a radicle that becomes the root and a plumule that becomes the shoot. With water, oxygen and warmth the seed germinates: the radicle grows down first, then the plumule grows up and the first leaves open.bean seedseed coatcotyledon (food store)radicle (root)plumule (shoot)germinating seedlingneeds water, oxygen and warmth

Primary and secondary growth

Primary growth makes the plant longer, at the shoot and root tips (apical meristems). Secondary growth makes stems and roots thicker in woody dicots: the vascular cambium produces secondary xylem (wood) inward and secondary phloem outward, and the cork cambium makes the bark. Annual rings in wood show the yearly growth.

Primary and secondary growth in a stemPrimary growth lengthens a stem at the tip, where the apical meristem divides. A young dicot stem has vascular bundles in a ring. Secondary growth thickens the stem: the vascular cambium between the xylem and phloem divides to add new xylem inwards and new phloem outwards, and a cork cambium makes the outer bark.pithyoung stem (primary)bundles in a ringolder stem (secondary)bark (cork)phloemcambiumxylem (wood)
Try itSeed viability

The viability of seeds falls with storage time. Choose the storage time and how fast the seeds die.

Check yourself In which phase is the DNA copied?
Check yourself What makes a stem thicker?
Check yourself Which treatment may end seed dormancy?

Key terms

Cell cycle
The sequence of events from one cell division to the next.
Meristem
A region of dividing cells.
Dormancy
A resting state in which a seed does not germinate.
Viability
The ability of a seed to germinate.

Summary

  • Mitosis makes two identical cells; DNA is copied in S phase.
  • Growth follows an S-shaped curve.
  • Dormancy is ended by scarification, cold or gibberellin.
  • Primary growth lengthens; secondary growth thickens.
Seed dormancy and viability