The Growth of the Shoot Apex in Maize: Embryogeny

The dimensional changes of the shoot apex are traced through embryogeny in an inbred strain of Zea mays L. The shoot apex passes through 5-6 plastochrons during embryogeny. Average length, width, and median sagittal area of the shoot apex increase regularly from stage 1 to stage 5. The change within each is greatest in stage 1 and least in stage 5, the maximum size increasing only slightly, the minimum markedly. 2. Cell size remains essentially constant through all five stages during embryogeny. Therefore the increase in shoot apex size, both within the plastochron, and over a succession of stages, is a function of increase in cell number. 3. The duration of plastochrons 1-5 is progressively greater, forming an exponential sequence. Thus, the rate of leaf initiation is deceleratory. 4. The rate of increase in median sagittal area of the shoot apex and thus in total cell number in the shoot apex from stage 1 to stage 5 is deceleratory. It follows that the specific growth rates for these two growth phenomena are also deceleratory. 5. The results of this investigation are compared with similar processes during seedling growth in maize. The constancy of cell size appears common to both phases of plant growth as well as the general increase in shoot apex size from stage to stage. However the rates of increase in shoot apex size and cell number, as well as rate of leaf initiation, are diametrically opposed in the two phases, being deceleratory during embryogeny and acceleratory during seedling growth.