Plant Growth and Development

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 13.

Why do you stop growing while an oak tree never does?

Your height is fixed by your late teens. A banyan tree, by contrast, adds new branches, leaves and roots for hundreds of years. The difference is not about size, it is about where cell division is allowed to continue. Plants keep small zones of dividing cells for life, and animals mostly do not. This chapter explains how those cells grow, how they become specialised, and how chemical messengers called plant hormones keep the whole programme in order.

What is growth, and where does it happen?

Growth is an irreversible, permanent increase in the size of an organ, its parts or even a single cell. It is generally accompanied by metabolic processes (building up and breaking down) that use energy. The word irreversible matters: a dry seed that swells in water or a piece of wood that soaks up water has only taken on water, and will shrink again, so that is not growth.

Plant growth is indeterminate, also called the open form of growth, because plants keep meristems (regions where cells divide and self perpetuate) active throughout life. Two kinds of meristem do different jobs.

At the cell level, growth is mainly an increase in protoplasm, which is hard to measure directly. So we measure fresh weight, dry weight, length, area, volume or cell number. The scale can be startling: one maize root apical meristem produces more than 17,500 new cells per hour, while a single watermelon cell can enlarge up to 3,50,000 times.

Three phases in a root tip. Moving back from the tip of a growing root you pass through three zones.

Patterns of growth: line or curve?

How a plant part grows over time falls into two patterns.

In the arithmetic formula, Lₜ is length at time t, L₀ is the starting length and r is the growth rate (elongation per unit time). In the geometric formula, W₁ is the final size, W₀ the starting size, r the relative growth rate and e the base of natural logarithms. Here r is also called the efficiency index, because it measures how efficiently the plant produces new material.

The sigmoid curve. It has three phases. A slow lag phase comes first. Then a rapid exponential (log) phase follows, when cells are dividing at the maximum rate, with plenty of nutrients. Finally the stationary phase sets in, when the rate slows because nutrients become limiting. This S shape is characteristic of the cells, tissues and organs of a plant.

Absolute and relative growth rate. The absolute growth rate is the total growth per unit time. The relative growth rate is the growth per unit time expressed on a common basis, such as per unit of the starting size. Take two leaves: leaf A grows from 5 cm² to 10 cm², and leaf B from 50 cm² to 55 cm². Each gains 5 cm², so their absolute growth rates are the same. But A has doubled (5 ÷ 5, or 100 per cent) while B has grown only 10 per cent (5 ÷ 50), so A has the much higher relative growth rate.

Pause and predict: a giant tree adds more wood each year than a sapling does. Does it follow that the tree is growing faster?

Check your answer: only in absolute terms. Relative to its existing size, the sapling is growing faster, just as leaf A beat leaf B above.

What does a growing plant need?

Differentiation, dedifferentiation and redifferentiation

Dividing cells are not the end of the story: a cell must also become something. Differentiation is the process by which cells formed from the root and shoot meristems and from the cambium mature into specialised cells. A cell that becomes a tracheary element, for example, loses its protoplasm and builds thick, strong, elastic lignocellulosic secondary walls, because it must carry water under tension.

Differentiation in plants is open: cells from the same meristem can end up with different structures depending on where they lie. Cells pushed away from the root apical meristem become root cap cells, while the peripheral ones mature into epidermis.

Development is the sum of growth and differentiation: the whole ordered succession of events from seed germination to death, which produces roots, leaves, branches, flowers, fruits and seeds. A seed germinates only when conditions are favourable. If they are not, it stays in a state of rest.

Plasticity. Plants may follow different developmental routes in response to their environment or phase of life. A familiar example is heterophylly, where leaves of different shapes occur on the same plant.

Beyond the textbook. For the exam: dedifferentiation produces meristems such as interfascicular cambium and cork cambium. In reality: the idea goes much further. Because many mature plant cells can restart division, a single cell can often be coaxed to form a whole plant, a property called totipotency, and this is the principle behind plant tissue culture. Why it matters: it shows that differentiation in plants is not a one way street, which is also why a cutting can grow roots.

Plant growth regulators: the chemical messengers

Plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition that control growth and development. They are also called plant growth substances or phytohormones. Their chemistry is a favourite for matching questions.

Functionally there are two groups. Auxins, gibberellins and cytokinins are growth promoters. ABA is a growth inhibitor and is involved in stress responses. Ethylene could fit either group, but it largely inhibits growth.

Five discoveries, mostly by accident

Beyond the textbook. For the exam: Darwin saw a transmittable influence in the tip and Went isolated auxin. In reality: the accepted explanation is that auxin in the coleoptile tip moves sideways towards the shaded side, and the cells there elongate faster, so the shoot bends towards the light. Why it matters: it turns a memorised discoverer name into an explanation of phototropism.

The five PGRs side by side

Auxins. Natural auxins are IAA and IBA, synthetic ones are NAA and 2,4-D. They help in four ways that appear again and again. They promote rooting in stem cuttings. They cause apical dominance: the growing apical bud suppresses lateral buds, and removing the tip (decapitation) lets the laterals grow, which is why tea bushes and hedges are pruned. They induce parthenocarpy, fruit formation without fertilisation, as in tomato. And 2,4-D works as a weed killer, killing dicot weeds without harming mature monocot crops. Auxins also promote flowering in pineapple, prevent early fruit and leaf drop, promote the dropping of older mature leaves and fruits, and help in xylem differentiation and cell division.

Gibberellins. They increase length: they lengthen grape stalks and improve the shape of apples. They delay senescence, so fruit can stay on the tree longer. They speed up malting in the brewing industry. Sprayed on sugarcane, they lengthen the stems, raising yield by as much as 20 tonnes per acre. They hasten the maturity of juvenile conifers, so seeds come sooner. And they promote bolting, the sudden elongation of internodes just before flowering in rosette plants such as beet and cabbage.

Cytokinins. These promote cell division and are made where it is most rapid. They help in producing new leaves, chloroplasts, lateral shoots and adventitious shoots. They overcome apical dominance, the opposite of auxin's effect on buds. They also mobilise nutrients to the leaf, which delays leaf senescence.

Ethylene. It is the only gaseous PGR and one of the most widely used in agriculture. In seedlings it causes horizontal growth, swelling of the axis and the apical hook of dicots. It promotes senescence and abscission of leaves and flowers. It is highly effective in fruit ripening, where it speeds up respiration, an event called the respiratory climacteric. It breaks seed and bud dormancy, starts germination in peanut and sprouting in potato tubers. It lets deep water rice stems grow quickly so that leaves stay above water. It also promotes root growth and root hairs, and induces flowering in mango and synchronised flowering and fruit set in pineapple. The common source is ethephon, which hastens ripening in tomato and apple, speeds abscission (thinning in cotton, cherry and walnut) and promotes female flowers in cucumber.

Abscisic acid. ABA is a general growth inhibitor and also inhibits metabolism and seed germination. It closes stomata under water stress and raises tolerance to stress, hence its nickname. It also helps seed development, maturation and dormancy so that seeds can withstand drying. In most situations it opposes gibberellins.

Pause and predict: you place one ripe banana in a paper bag with a hard, green mango. Why does the mango ripen sooner?

Check your answer: the banana releases ethylene, a gas that diffuses to the mango and triggers ripening. That is exactly what Cousins observed with oranges and bananas.

Pause and predict: a gardener cuts off the top bud of a tea bush. What happens to the side buds, and which hormone's effect has been removed?

Check your answer: they start to grow into branches, because the source of auxin that kept them suppressed (apical dominance) has been removed.

Hormones work as a team. PGRs may act together or against each other, and one process usually involves more than one. Dormancy, abscission, senescence and apical dominance are examples. PGRs are also only one kind of control, working along with the plant's genes and with outside factors such as temperature and light. Those outside signals often act through PGRs to control vernalisation, flowering, dormancy, seed germination and plant movements.

Beyond the textbook. For the exam: gibberellins cause bolting and increase stem length. In reality: the bakanae disease is a hormone overdose: the fungus makes gibberellin and the infected rice seedlings grow tall and weak. The fungus is now classified under the name Fusarium fujikuroi. Why it matters: it is a clean example of a plant hormone being found first through a disease.

Exam traps

60-second revision

Where this chapter connects

• Previous: Respiration in Plants supplies the energy for growth, and ethylene is why respiration rises as fruit ripens.
• Next: Anatomy of Flowering Plants shows the tissues made by the apical and lateral meristems met here, including secondary growth.
• Class XII link: Sexual Reproduction in Flowering Plants covers seed dormancy, fruit development and parthenocarpy, all controlled by the hormones in this chapter.