Summary

Growth stands out as one of the most visible features of any living thing. It shows up as an irreversible rise in measurable quantities — size, area, length, height, volume or cell number — and at its core is an accumulation of protoplasmic material.

In plants specifically, growth is centred at the meristems: the root and shoot apical meristems, sometimes joined by an intercalary meristem, drive the lengthwise growth of the plant axis, and unlike in most animals, this growth in higher plants never truly stops — it is indeterminate. Once meristem cells divide, the resulting growth can proceed either arithmetically or geometrically. It rarely holds one fast rate throughout a cell's, tissue's, organ's or organism's whole life — growth instead moves through three recognisable phases: a slow lag phase, a fast log phase, and an eventual senescent phase.

Once a cell stops dividing, it moves into differentiation, taking on a structure suited to whatever function it will finally perform — the underlying rules of differentiation are much the same whether the structure in question is a cell, a tissue or an organ. A differentiated cell is not necessarily locked in: it can dedifferentiate and later redifferentiate. Because plant differentiation stays open in this way, development itself stays flexible — which is exactly why development is best understood as growth and differentiation combined, and why plants show such marked plasticity as they develop.

Growth and development in plants are steered by both intrinsic and extrinsic factors. The chief intercellular intrinsic factors are the chemical substances called plant growth regulators (PGRs), which fall principally into five groups: auxins, gibberellins, cytokinins, abscisic acid and ethylene — each synthesised in various parts of the plant, each steering a different set of differentiation and developmental events. …