Growth and development: principles, patterns and plant growth

Learning objectives

By the end of this lesson, you should be able to:

• distinguish growth from development
• describe how growth occurs in plants and animals
• explain the roles of cell division, cell enlargement and cell differentiation
• identify regions of growth in flowering plants
• describe primary and secondary growth
• interpret a typical growth curve

Meaning of growth and development

Growth is a permanent increase in the size, dry mass, volume or number of cells of an organism. It results mainly from cell division and cell enlargement. A temporary increase in size caused only by water intake is not, by itself, reliable evidence of growth.

Development is the series of changes through which an organism becomes more complex and functionally mature. It includes cell differentiation, formation of tissues and organs, and changes in body form and function.

Growth and development are related but are not identical. A seedling increases in height and mass as it grows; at the same time, it develops leaves, roots and specialised tissues. Some developmental changes, such as the formation of a flower, may occur without a large increase in the organism's overall size.

How growth occurs

Growth in multicellular organisms involves three linked processes:

1. Cell division: one cell divides to produce new cells. In ordinary growth of body tissues, this is usually mitosis.
2. Cell enlargement: new cells increase in size through synthesis of cytoplasm and other cell materials. In plants, water uptake and vacuole expansion contribute substantially to cell enlargement.
3. Cell differentiation: cells become specialised in structure and function. For example, some plant cells develop into xylem vessels, while others form phloem or photosynthetic tissue.

Cell division increases cell number; enlargement increases the size of individual cells; differentiation produces the specialised tissues needed for normal function.

Growth in plants

Plant growth is concentrated in regions called meristems. Meristematic cells are relatively unspecialised, have thin walls and dense cytoplasm, and divide actively by mitosis.

Apical meristems and primary growth

Apical meristems occur at the tips of roots and shoots. Their activity increases the length of roots and stems; this is called primary growth.

At a shoot tip, cell division produces new cells that later elongate and differentiate into tissues such as epidermis, cortex, vascular tissue and leaves. At a root tip, the apical meristem lies behind the protective root cap. New cells contribute to root length and form specialised root tissues.

The root cap protects the delicate growing tip as it pushes through soil. The zone of cell division is followed by a zone of elongation, where cells increase in length, and a zone of maturation or differentiation, where cells become specialised. Root hairs develop in the maturation region and increase the surface area for absorption of water and mineral salts.

Lateral meristems and secondary growth

In many dicotyledonous plants and gymnosperms, lateral meristems increase the thickness or girth of stems and roots. This is called secondary growth.

The vascular cambium produces secondary xylem towards the inside and secondary phloem towards the outside. Secondary xylem forms much of the wood in a mature woody stem. The cork cambium produces protective cork cells that contribute to the outer bark.

Many monocotyledonous plants, such as grasses, do not normally show the same typical secondary thickening as woody dicots.

Conditions needed for plant growth

Plants need water for photosynthesis, transport, turgor and cell expansion. Mineral ions are required to make proteins, chlorophyll, nucleic acids and other cell materials. Light supplies energy for photosynthesis. Carbon dioxide is a raw material for photosynthesis, while oxygen is needed for aerobic respiration. Suitable temperature is necessary for enzyme-controlled reactions.

Plant hormones regulate growth and development. Auxins promote cell elongation in many shoot tissues and are involved in responses such as phototropism. Gibberellins can stimulate stem elongation and seed germination. Cytokinins promote cell division. Abscisic acid is associated with responses to water stress and maintenance of seed dormancy. Hormone effects depend on concentration, tissue and developmental stage.

Growth in animals

Animal growth also involves mitosis, cell enlargement and differentiation, but unlike most plants, many animals have a more limited distribution of actively dividing growth regions. In mammals, growth occurs in many tissues during development. In long bones, lengthening before maturity occurs at cartilage growth plates near the ends of the bones; these plates eventually close.

Animal growth may be relatively rapid during early life, slow during maturity and cease or become very limited in some tissues after adulthood. Some animals, including many fish and reptiles, can continue growing after reaching reproductive maturity, although the rate often decreases.

Growth curves

A growth curve shows how a measured feature, such as height, length, mass or cell number, changes with time.

A typical curve for many organisms under suitable conditions is sigmoid (S-shaped):

• Lag phase: growth is initially slow while cells become established and metabolic activity increases.
• Exponential or rapid phase: growth becomes faster as cells divide and enlarge actively.
• Decelerating phase: the rate of growth slows as resources, space or other conditions become limiting, or as the organism approaches maturity.
• Stationary or maturity phase: the measured size changes little over the period observed.

Not every organism or measured feature produces a perfect S-shaped curve. Some plants show seasonal growth, and different organs can have different growth patterns.

WASSCE examination focus

• Define growth as a permanent increase in size, dry mass or cell number; distinguish it from development.
• If asked where a root increases in length, identify the apical meristem near the root tip.
• Do not confuse primary growth (increase in length) with secondary growth (increase in girth).
• Explain that meristematic cells divide, newly formed cells enlarge, and cells later differentiate.
• When interpreting a growth curve, describe the rate of growth in each phase rather than merely naming the curve.

Quick check

1. Distinguish growth from development.
2. Name the type of cell division mainly responsible for growth in a plant.
3. Where are apical meristems found?
4. State the difference between primary and secondary growth.
5. Name the tissue that produces secondary xylem.
6. What happens during the exponential phase of a sigmoid growth curve?

Answers

1. Growth is a permanent increase in size, dry mass, volume or cell number; development is the sequence of changes that produces greater complexity and maturity.
2. Mitosis.
3. At the tips of roots and shoots.
4. Primary growth increases length; secondary growth increases girth or thickness.
5. Vascular cambium.
6. The organism grows rapidly as cells divide and enlarge actively.