An Automata-theoretical Model of Meristem Development as Applied to the Primary Root ofZea maysL.

Abstract

Observations were made of the sequence of division within the cellular packets (groups of cells of common descent) which comprise the cell files that run the length of the central cortex of the primary root meristem ofZea mays.These sequences, and also the relative lengths of the cells within the packets recorded at various times during root growth, 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. The outcome is a regular pattern of divisions from which daughter cells emerge usually with unequal, but definite, lengths. In the immediately post-germination stage of root growth, one division pathway is especially common in the cortex and generates sequences of unequal daughters having a particular basi-apical orientation. Later in root growth, the cellular pattern in the cortex indicates that this pathway is replaced by another where unequal divisions are not so marked, but which nevertheless continues to maintain a regular arrangement of differently sized cells. This latter pathway is characteristic of a zone close to the initial cells of the cortex. It is present at all stages of root growth 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 bootstrap systems. The files so generated contain all the observed cell patterns. The growth functions also predict the sequence in which cells cease dividing near the proximal margin of the meristem, but for this it is necessary to incorporate a counter for the number of divisions that will be accomplished in the cell file. Cytological requirements for the propagation of unequal divisions, together with a consideration of the nature of the division counter, as well as the significance of the switch in division pathways encountered during early root growth, are discussed in the context of this deterministic model of cell division.