Phases and Rate of Plant Growth

Growth, in the strict physiological sense used throughout plant biology, is an irreversible, permanent increase in the size of a cell, an organ, or the whole plant, and it is measured in a variety of practical ways depending on convenience -- increase in fresh weight, dry weight, length, area, volume, or simply the number of cells present. A defining feature of plant growth, quite unlike growth in most animals, is that it is indeterminate (open-ended): a plant does not stop growing once it reaches some fixed adult size but instead continues adding new cells and organs throughout its life, because it retains localised regions of permanently dividing cells -- the meristems -- at its growing points, something most animal bodies lose once embryonic development is complete.

A cell produced by an apical meristem normally passes through three overlapping phases on its way to becoming a mature, functional cell. In the meristematic phase, at the very tip of the root or shoot, cells are small, densely packed with cytoplasm, have a large nucleus relative to the cell's small size, thin cellulosic walls rich in plasmodesmatal connections to their neighbours, and divide actively and repeatedly. Just behind this zone lies the elongation phase, where cells are no longer dividing but instead enlarge rapidly, chiefly through the uptake of water into an expanding central vacuole, while new cell wall material is laid down to accommodate the increase in volume; this phase contributes the great bulk of the visible lengthening of a growing root or shoot. Finally, in the maturation phase, cells stop enlarging, the protoplasm is proportionally reduced as the vacuole comes to occupy most of the cell's volume, and the cell wall is often further thickened and chemically modified (for example, lignified in a tracheary element) as the cell settles into its final, specialised, mature form. …