AN AUTOMATA-THEORETICAL MODEL OF MERISTEM DEVELOPMENT AS APPLIED TO THE PRIMARY ROOT OF ZEA-MAYS L

Observations were made of the sequence of division within the cellular packets (groups of cells of common descent) which comprise the cell f iles that run the length of the central cortex of the primary root mer istem of Zea mays. These sequences, and also the relative lengths of t he cells within the packets recorded at various times during root grow th, indicate that cell-file development can be expressed using one, or a limited number, of deterministic 'bootstrap' L-systems which assign different lifespans to sister cells of successive cell generations. T he outcome is a regular pattern of divisions from which daughter cells emerge usually with unequal, but definite, lengths. In the immediatel y postgermination stage of root growth, one division pathway is especi ally common in the cortex and generates sequences of unequal daughters having a particular basi-apical orientation. Later in root growth, th e cellular pattern in the cortex indicates that this pathway is replac ed by another where unequal divisions are not so marked, but which nev ertheless continues to maintain a regular arrangement of differently s ized cells. This latter pathway is characteristic of a zone close to t he initial cells of the cortex. It is present at all stages of root gr owth and spreads along the length of the cortex as the descendants of these initials proliferate. The development of the whole cortical cell file can be simulated from knowledge of the growth functions of the b ootstrap systems. The files so generated contain all the observed cell patterns. The growth functions also predict the sequence in which cel ls cease dividing near the proximal margin of the meristem, but for th is it is necessary to incorporate a counter for the number of division s that will be accomplished in the cell file. Cytological requirements for the propagation of unequal divisions, together with a considerati on of the nature of the division counter, as well as the significance of the switch in division pathways encountered during early root growt h, are discussed in the context of this deterministic model of cell di vision. (C) 1997 Annals of Botany Company.