The extension in a maize leaf phytomer was studied in terms of the one dimensional displacement of its fabric relative to its base as a function of position and of age. The relationship between displacement, position and age was termed the displacement field. The displacements, during a 24 hour interval, of a series of points spaced along the length of a particular phytomer was recorded. This provided an Eulerian evaluation of the 24 hour displacement field. A population of plants each representing a different age were used and the average displacement field of the phytomer during its period of growth was established.
Extension was localised in the region named the growth zone at the base of the phytomer. The length of the growth zone rose and fell following a wave shaped curve with respect to age. It was never constant. The relationship between the displacement of and position of points within the growth zone formed a smooth sigmoid curve. The presence of the ligule and of the node of leaf insertion did not give rise to any local reduction in extension.
A method of tracing the movement of a point in the leaf fabric, through the growth zone, over a period of time is explained. Such Lagrangian pathlines are described using referential notation, X=x(t;x.). The pathlines of points were constructed by a stepwise addition of the relevant displacement values read from the displacement field for each successive position and age.
The pathlines of two chosen points provided positions, as a function of age, for the boundaries of a growing and moving segment. The segment was in mid lamina. Its length increased from 0.025mm when its lower boundary was at 3.6mm from the leaf base to 2mm at maturity. The numbers by types of cells in the lower epidermis of the segment were surveyed at a series of plant ages. There was an initial decrease and then a considerable increase in the average length of the non stomata! cells during the period of the experiment. During the same period the segment increased in length and this was accompanied by an increase in the number of cells in the sample rows. This increase in cell numbers was continuing when the segment reached a position 40mm
from the base of the leaf within the then 67mm long growth zone. The associated cell division was occurring in all of the cell categories, not the stomata! complexes alone. The development of stomata with respect to plant age is discussed. Stomata! initiation in the leaf segment was not restricted to a single occasion or unique position in the growth zone.