THE BRANCH ROOTS OF ZEA .2. DEVELOPMENTAL LOSS OF THE APICAL MERISTEM IN FIELD-GROWN ROOTS

Most of the first-order branches of field-grown maize roots are short (around 10 mm). An anatomical and morphological survey of the tips of branches from nodal roots showed that almost all branches lacked a normal apex. Most tips were broken off (e.g. 74% in the mid-regions of roots from node 1, 95% in the same region of node 3 roots), with living, mature tissues right to their end. The remaining tips were of 3 types: (a) Determinate tips with rounded off ends, no root cap, and mature tissues right at the tip; (b) Normal tips which were miniaturized versions of main root apices with a meristem, root cap and long zone of elongation; (c) Intermediate tips in which cap loss, meristem outgrowth and differentiation were incomplete. Normal tips were mainly on branches of the youngest roots and determinate tips were the predominant intact tips of old roots. This distribution, together with anatomical features of the three tip types indicates that Determinate tips form from Normal tips by a controlled pattern of cell enlargement, differentiation and, possibly, cell division. Many tips must be broken accidentally by excavation, or earlier by browsers or pathogens. There was some evidence, however, that some broken tips are formed by developmentally controlled abscission both before and after the formation of determinate tips.