CHANGES IN VOLUME AND CELL NUMBER IN DIFFERENT REGIONS OF SHOOT APEX OF PISUM DURING A SINGLE PLASTOCHRON

The length of the ninth plastochron in shoot apices of Pisum sativum was measured and found to be almost 46 h. This single plastochron was divided into 11 morphologically recognizable stages and the time taken to reach each stage was measured. The cell number and cell volume of five regions of the apex was measured at each stage of the plastochron. Although the apex as a whole grew exponentially, growth during the first 30 h of the plastochron was predominantly in the primordium and the adjacent tissues, whereas in the last 16 h growth was mainly in the apical dome. Since the mean cell volume remained constant, different rates of growth were due to different rates of cell division. The data suggested that the apex probably grows by the formation of growth centres on alternate sides of the apex, the beginning of each new growth centre being apparent as an increased rate of growth in the apical dome 16 h before the beginning of the next plastochron. The inception of a new primordium may therefore precede its appearance as a bump by about 16 h, and precede the first periclinal division in the tunica by 26 h. A central zone of larger cells with lightly-staining nuclei was found at the extreme apex. This central zone became reduced in size or disappeared at the time at which a new primordium was about to become visible. © 1968 OXFORD UNIVERSITY PRESS.