Fig. 6.8-3. Longitudinal section through the zone of elongation just proximal to the root apex of cattail (Typha). Notice the tremendous differences in cells size. The very large cells in the two columns in the center will become vessel elements in the xylem, the surrounding smaller cells will differentiate into a variety of vascular tissues. The main point illustrated here is that as a given block of living tissue moves away from the apical meristem and expands, it will form a few large cells if cell division is slow or many small cells if it is rapid. For example, two small cubes of meristem tissue measuring 10mm on each side might both expand to become blocks of mature tissue measuring 1000mm on each side, but one may be divided up into many small cells, the other into just a few large cells. It is easy to assume that a large cell is larger because it is growing faster than adjacent small cells, but that would be incorrect.

Each of the large vessel element precursor cells is surrounded by many smaller cells, but because they are all growing at the same rate, the walls of the larger cells are not sliding past the walls of the smaller cells (as would be necessary if the smaller cells were smaller because they were growing more slowly). Because all parts of this tissue are growing at the same rate, it is said to be symplastic growth.