Plant Growth and Development

Growth is defined as an irreversible permanent increase in the size, mass or number of cells of an organism (or its parts). In plants it is brought about by cell division followed by cell enlargement, and it is fuelled by metabolism. A distinctive feature of plants is that they show indeterminate (open) growth throughout life, because they retain regions of perpetually dividing cells — the meristems — at their root and shoot tips.

Growth at the tips proceeds through three zones (phases) in sequence. The phase of meristematic growth is the tip itself, where cells divide repeatedly and are rich in protoplasm. Just behind it is the phase of elongation, where cells increase in size through vacuolation and the deposition of new wall material. Beyond that lies the phase of maturation, where the enlarged cells differentiate and attain their final size and form. Recognising these three phases in order is a standard NEET point.

The rate of growth — the increase per unit time — can follow two patterns. In arithmetic growth, after a cell divides only one daughter continues to divide while the other differentiates, giving a constant, linear increase (e.g. a root elongating at a steady rate). In geometric growth, both daughter cells keep dividing, so growth is exponential at first; in practice it produces the classic sigmoid (S-shaped) growth curve with three parts — a slow lag phase, a rapid log (exponential) phase, and a levelling stationary phase as nutrients become limiting.

Growth can be measured in many ways — increase in length, area, volume, cell number, or fresh and dry weight — and the right measure depends on the system. Several conditions are essential for growth: an adequate supply of water (cells need turgor to enlarge), oxygen (for the energy of respiration), nutrients (raw materials), and a suitable temperature and light environment. For NEET, the key takeaways are the three growth phases, the arithmetic-versus-geometric distinction, and the sigmoid growth curve.

As cells leave the meristem and mature they undergo differentiation — they take on specific structures and functions suited to a particular role. For example, cells that become the water-conducting tracheary elements of the xylem lose their protoplasm and develop strong, lignified secondary walls. Differentiation thus converts simple meristematic cells into the many specialised cell types of the plant body.

Remarkably, plant cells can reverse and re-reverse this. Dedifferentiation is when already-differentiated cells that had stopped dividing regain the ability to divide. Familiar examples are the formation of the interfascicular cambium and the cork cambium from fully mature parenchyma cells during secondary growth. Redifferentiation is the opposite final step: these dedifferentiated cells mature once more and again lose the capacity to divide, becoming specialised. Distinguishing these three terms — differentiation, dedifferentiation, redifferentiation — is a guaranteed NEET question.

Taken together, development is the sum of all the changes an organism passes through during its life cycle, from seed germination to senescence; in short, development = growth + differentiation. The same genes can produce different outcomes depending on internal (intrinsic) and external (environmental) signals, which is why a plant can adjust its body to its surroundings.

This adaptability is called plasticity. A clear illustration is heterophylly — the production of different leaf shapes on the same plant under different conditions. In cotton, coriander and larkspur the leaves of the juvenile plant differ from those of the mature plant; and in an aquatic plant such as buttercup, the leaves formed under water are finely dissected while those in air are broad. Plasticity shows how growth and differentiation are tuned by both the plant's age and its environment, completing the picture of plant development.

Plant growth regulators (PGRs), also called phytohormones, are small chemical molecules that control growth and development. They fall into two broad groups: growth promoters — auxins, gibberellins and cytokinins — and growth inhibitors — abscisic acid (ABA) and ethylene (ethylene has mixed roles but is grouped with the inhibitors). Knowing the signature function and a key example for each is the single most-tested part of this chapter.

Auxins (the natural one is IAA) were the first to be discovered, from the Darwins' work on coleoptile bending toward light and Went's isolation from oat coleoptile tips. Auxins promote cell elongation, maintain apical dominance (the suppression of lateral buds by the shoot tip), initiate rooting in cuttings, induce parthenocarpy (seedless fruit, e.g. tomato), prevent premature abscission, and as the synthetic 2,4-D act as a weedkiller selective for broad-leaved (dicot) weeds. Gibberellins (GA₃) cause bolting (rapid internode/stem elongation in rosette plants), increase the length of axes and the size of fruit (e.g. grapes), help in malting in the brewing industry, and break seed and bud dormancy.

Cytokinins (such as kinetin and zeatin) promote cell division (cytokinesis), help overcome apical dominance and promote lateral shoot growth, aid the mobilisation of nutrients, and notably delay leaf senescence (ageing) — the Richmond–Lang effect. Among the inhibitors, ethylene is a gaseous hormone that promotes fruit ripening, senescence and abscission of leaves and flowers, breaks dormancy, and induces flowering in pineapple and mango.

Abscisic acid (ABA) is the great inhibitor, well known as the 'stress hormone'. It induces and maintains seed dormancy, inhibits growth, and — crucially for surviving drought — triggers the closure of stomata during water stress; it generally acts as an antagonist of the gibberellins. The most efficient way to revise is a single line per hormone: auxin = apical dominance + 2,4-D; GA = bolting + malting; cytokinin = cell division + delays senescence; ethylene = ripening + abscission; ABA = stress, stomatal closure, dormancy.

Flowering is not triggered by hormones alone; many plants also depend on environmental cues, chiefly the length of day and the temperature. Photoperiodism is the response of flowering to the relative lengths of the light and dark periods — the photoperiod. On this basis plants are classified into three groups, a guaranteed NEET fact.

Short-day plants (SDPs) flower only when the day length is below a critical value — i.e. when the night is long (e.g. chrysanthemum, soybean). Long-day plants (LDPs) flower only when the day length is above a critical value — when the night is short (e.g. spinach, henbane). Day-neutral plants (DNPs) flower regardless of day length (e.g. tomato, cucumber). It later emerged that the length of the dark period is in fact the critical factor, so a 'short-day' plant is really a long-night plant.

A key detail often tested is the site of perception: the photoperiod is sensed by the leaves, not by the shoot tip where flowers form. The leaves are thought to produce a flowering hormone (historically named florigen) that travels to the shoot apex and switches on flowering. Thus a plant can be made to flower or kept vegetative simply by controlling the duration of light and darkness its leaves receive.

The second environmental control is temperature, through vernalisation — the promotion of flowering by a period of exposure to low temperature (cold). Vernalisation prevents plants from flowering before winter is over and is required by many winter varieties of cereals (e.g. winter wheat, rye) and by biennials such as sugar beet, cabbage and carrot. Together, photoperiodism and vernalisation let plants time their flowering to the most favourable season. For NEET, remember: SDP/LDP/DNP definitions, that the leaf perceives the photoperiod, and that vernalisation is a cold requirement for flowering.