Changes in cell wall organization resulting from surface growth in parenchyma of oat coleoptiles

The primary walls present during the phase of extension growth in oat coleoptiles possess an almost transverse microfibril orientation on their inner surfaces but on the outer surface the microfibrils are considerably disoriented from this direction, which is consistent with the concept of multi-net mechanism of growth. Coleoptile segments grown at 2°C to depress cell wall formation show no difference in orientation on their inner and outer surfaces; this is also considered to be consistent with the multi-net mechanism. It is shown that the longitudinal ribs of microfibrils present at the cell corners, and hitherto referred to as secondary thickening, are on the outer surface of the cell wall and are considered to arise from a disorientation of microfibrils as a result of multi-net growth. As a result of this microfibril disorientation there is a tendency for the pit fields to be reduced in area. After surface growth has ceased a secondary wall is formed with a well-defined helical organization distinctly different from that of the primary wall. The implications of these results in terms of previous investigations are discussed.