22.2Cell Division and Growth
Development and growth in all multicellular organisms, not just plants, is supported by the increase in the number of cells through cell division. However, plant cells are fixed in place, that is, they cannot move, and are also mutually bound at the cell wall. For this reason, there is virtually no change in their positional relationship with other cells after birth. Thus, in the development and growth of plants, when and where cells proliferate hold particular importance.
Except for the embryonic stage, cell division is concentrated only in certain areas of the plant. Such areas include the shoot apical meristem, which occurs in the apical and lateral buds; the root apical meristem, which lies at the tip of the root; the cambium, which is a lateral meristem of the stem and root; and the intercalary meristem found in the internodes and the base of leaves of gramineous plants. Because such cell division occurs locally and is responsible only for local growth at the respective sites, most of the plant body remains stationary. In other words, plants grow by adding newly built parts to the older parts of their body that remain in the original form (just like extending a house).
The growth in the size of individual cells is as important as the increase in the number of cells in plant development and growth. In fact, plant cells undergo a conspicuous increase in volume through the development of vacuoles. The rapid cell growth accompanying vacuolization usually occurs after cell division stops. At the root apex, shoot apex, and in the leaves of gramineous plants, this process can be seen in the spatial arrangement of cells, in which the zones of cell division and cell elongation lie next to each other (Figure 22-2). This phenomenon plainly shows that cell division and associated cell growth (elongation of individual cells) drive the growth of plant organs .
Figure 22-2 Relationships between cell division and cell growth
These figures illustrate the division and elongation growth of epidermal cells in corn leaves. In the corn leaf, cells are produced in the intercalary meristem at the base, and elongation is accelerated as the cells move away from the base increases.
Furthermore, the orientation of cell growth plays a part in organ morphogenesis. In other words, the direction in which cells grow in respect to the axis of organs has a large influence on the shape of the whole organ. Thus, the direction of cell growth serves as a principal control point in which plants change organ growth according to the circumstances they are placed in. This topic is discussed in section 5 of this chapter in relation to stem growth.