Growth, Differentiation and Development: Quick Revision

A brief note on Growth, Differentiation and Development. Read it before a test. Try each question at the end before you open its answer.

Growth

• Growth: a permanent, irreversible increase in size, made by living cells using energy. Swelling of dry wood in water is not growth.
• Plant growth is indeterminate and open, because the meristems keep adding cells.
• Measures: fresh weight, dry weight, length, area, volume and cell number. Dry weight is better than fresh weight.
• Maize root apex: more than 17,500 cells an hour (cell number). Pollen tube: length only. Flat leaf: area.

The three phases of growth

Growth rates

• Arithmetic growth: one daughter cell divides again. Straight line. $L_t = L_0 + rt$.
• Geometric growth: both daughter cells divide again. Sigmoid curve: lag, log, stationary. $W_1 = W_0 e^{rt}$.
• $r$ is the relative growth rate, also called the efficiency index.
• Absolute growth rate: total gain in a unit of time. Relative growth rate: gain per unit of the starting size.
• A part that doubles in a time $t_d$ has $r = \dfrac{0.693}{t_d}$.
• Conditions: water, oxygen, nutrients, suitable temperature. Light and gravity act as signals.

Differentiation and development

• Differentiation: cells made by the meristems mature to do a job. It is open: the place of a cell decides its form.
• Dedifferentiation: living permanent cells divide again, as in the interfascicular cambium and the cork cambium.
• Redifferentiation: the cells of the new meristem mature again.
• Development = growth + differentiation, from germination to senescence.
• Plasticity: heterophylly by age in cotton, coriander and larkspur, and by surroundings in buttercup.
• Intrinsic factors: genes and growth regulators. Extrinsic factors: light, temperature, water, oxygen, nutrition.

A worked example

Leaf A grows from $5\ \text{cm}^2$ to $10\ \text{cm}^2$ in a week. Leaf B grows from $20\ \text{cm}^2$ to $25\ \text{cm}^2$. Compare them.

1. Both gain $5\ \text{cm}^2$, so the absolute growth rate is the same.
2. Leaf A: $\dfrac{5}{5} \times 100 = 100\%$ a week. Leaf B: $\dfrac{5}{20} \times 100 = 25\%$ a week.
3. Leaf A has the higher relative growth rate.

Practice with new numbers

A program to try

The program prints the absolute and the relative growth rate of two leaves.

Mistakes to avoid

• Calling every swelling growth. Growth is permanent.
• Using fresh weight as the best measure. Dry weight is better.
• Mixing arithmetic and geometric growth. One daughter cell divides again in arithmetic growth, and both do in geometric growth.
• Saying equal absolute rates mean equal relative rates. Compare with the starting size.
• Putting buttercup with age. Its leaf form depends on the surroundings.

Check yourself

1. What are the three phases of growth in a root, from the tip?

2. Which kind of growth gives a straight line?

3. Name the three phases of the sigmoid curve.

4. Give one example of dedifferentiation.