ORIGIN OF THE QUIESCENT CENTRE IN ZEA MA YS

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Abstract
SUMMARY: Establishment of the quiescent centre has been studied in embryogeny, in development of lateral root primordia and in roots regenerating after decapitation. In embryos a quiescent centre first appears just after the oblique root‐shoot axis becomes discernible, as soon as a cap meristem forms at the root pole and then again during the early stages of germination of seeds. In lateral primordia a quiescent centre also arises twice; once while the first set of cap initials, which are of endodermal origin, is functioning and then after they have ceased operating when new cap initials of pericyclic origin have replaced them. The first quiescent centre exists while the primordium is still embedded in the mother cortex, but the second can occur either before or after emergence from the mother root. In regenerating roots a quiescent centre establishes itself when a cap meristem becomes transiently distinct from the rest of the meristem by a difference in planes of cell growth and division. This happens before the meristem acquires the characteristic cell pattern of a normal grass root.These observations are discussed in relation to the reasons for the existence of quiescent centres to root meristems.