Concepts: Differentiation, development and plasticity

2.1 From dividing cells to working cells

Cells produced by a meristem are small, thin walled and structurally simple. To become part of a working tissue they change shape, develop a large vacuole, thicken their walls and take on a particular function. This change is differentiation, and it is the step that turns a mass of dividing cells into the tissues of a leaf, a root or a stem. The plant tracheary elements are a standard example: they lose their protoplasm and develop strong lignified walls so that they can carry water.

2.1.1 Dedifferentiation

A mature living cell is not always fixed in its role. In some situations a differentiated cell can lose its special features and start dividing again. This is dedifferentiation, and it is the reason interfascicular cambium and cork cambium can form in a stem that has already finished its primary growth. Plant tissue culture uses the same ability: a small piece of mature tissue placed on a suitable medium can form a callus, an undifferentiated mass of dividing cells.

2.1.2 Redifferentiation

A callus can be made to form roots or shoots when the medium is changed, and this recovery of a specialised structure from the undifferentiated mass is redifferentiation. The sequence meristem to differentiated tissue, then back to callus, then again to root or shoot, is the basis of micropropagation and of most plant tissue culture work.

2.1.3 Development and plasticity

Development is what happens when growth and differentiation work together. It covers every change that a cell, a tissue or a whole plant passes through as it ages and as its surroundings change. Plant development is called open because new organs are formed continuously, and it is called plastic because the same plant can produce different forms in response to its surroundings. Heterophylly is the clearest example: the larkspur plant and the buttercup produce different leaf shapes in different parts of the same plant. In the aquatic buttercup the submerged leaves are finely divided while the leaves on land are broad and lobed. The difference is not genetic, because the same plant can show both shapes; it is a response to water and light.

Plasticity has a practical side. Leaves on a shady side are thinner and wider than leaves on a sunny side, and the same species growing in a dry place may produce smaller, thicker leaves. Figure 13.8 arranges the whole sequence as a flow: meristematic cell, plasmatic growth, expansion and elongation, differentiation, maturation and finally senescence. The photographs of heterophylly in figure 13.9 sit alongside that sequence as the visible result of plastic development.

2.1.4 Growth and differentiation stay open

In higher plants growth and differentiation do not stop when the plant reaches a fixed size. Axillary buds can remain dormant for years and then grow into branches, and injured tissues can form new cells from surviving meristems. This open pattern is why a tree can be pruned and still produce new branches, and why a cutting can root and grow into a new plant.

2.1.5 The developmental sequence in one picture

Figure 13.8 puts the whole story in one flow. A meristematic cell first goes through plasmatic growth, the increase in cytoplasm that precedes any change in shape. Then it enters the expansion or elongation stage, where the vacuole enlarges and the wall stretches. Differentiation follows, and the cell takes on its specialised structure. Maturation completes the change, and the mature cell finally reaches senescence and death. The same sequence applies to a tissue or to a whole organ, which is why the figure is labelled for a cell but used in the chapter for development in general. The three terms are easy to mix in an exam, so keep their directions in mind: differentiation moves forward from simple cells to specialised ones, dedifferentiation moves back to a dividing state, and redifferentiation moves forward again from those dividing cells. Plasticity adds a second dimension, because the form that is produced can change with the environment even when the genetic material is the same. The slide collection beside this page shows figure 13.1 for the start of the story and figures 13.8 and 13.9 for the developmental sequence and its plastic outcome.

2.1.6 Practice from the NCERT exercises, questions 1 and 6

Exercise 1. Growth is an increase in size or mass through cell division and enlargement. Differentiation is the change by which meristematic cells become specialised. Development is growth and differentiation together over the life of the organism. Dedifferentiation is the return of a mature cell to a dividing state, redifferentiation is the recovery of a specialised structure from such dividing tissue, determinate growth is the growth pattern that stops after a fixed size, a meristem is a region of continuously dividing cells, and growth rate is the increase in size per unit time.

Exercise 6. In higher plants growth and differentiation are open because new cells and organs are produced throughout life from meristems, and the pattern of differentiation depends on position and environment rather than ending at a fixed body plan.