Abstract
Comparative cytohistological studies were made of shoot tips of Equisetum hyemale, Picea abies, and Prunus Persica, as examples from three major taxonomic groups of plants: lower vascular plants, gymnosperms, and angiosperms. The purpose of this study was to find out more specifically what factors determine the formation of apical histogenic layers. Obviously the plane of mitotic division in the cells at the upper surface of shoot apices determines the presence or absence of apical cell-layer formation. The hypothesis proposed is that the plane of mitotic division-positioning of metaphase chromosome plates-in cells in the surface region in shoot apices is determined by: 1) size of cells relative to shape and size of apical dome; 2) size of cells and space in cells relative to size of their nuclei and the freedom of their nuclei from wall pressure; and 3) location of cells and their shape and availability of space across a particular plane in cells in which metaphase plates can be accommodated. The hypothesis would explain the general predominance, or presence, of histogenic layers in shoot apices of angiosperms and relative paucity or lack of them in shoot apices of gymnosperms and lower plants.
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