Changes in microfibril orientation in the walls of elongation plant cells

The cell wall organization of sub-epidermal parenchyma of coleoptiles of Avena and the collenchyma of Apium has been investigated using staining techniques, freeze etching and the examination of shadowcast sections from which the embedding medium was removed. In the coleoptile parenchyma of Avena it has been confirmed that in the cell wall adjacent to the plasmalemma the microfibrils are transversely oriented and at the outer surface of the wall a dispersed arrangement of microfibrils is present. Longitudinally oriented microfibrils are present in the corner thickenings of the walls. There was no evidence of the crossed polylamellate structure described as being present in collenchyma; epidermal parenchyna; and subepidermal parenchyma of some roots and hypocotyls. The collenchyma of Apium showed the presence of wall layers in which the microfibrils were alternately of transverse and longitudinal orientation. In some layers microfibrils with a different orientation were present. The wall organization of the subepidermal parenchyma of Avena and the crossed polylamellate organization of Apium collenchyma are considered to be structural extremes of wall organization in elongating cells. The observations are discussed in relation to the 'multi-net growth’ hypothesis and the ‘ordered fibril’ hypothesis. It is concluded that the observed changes in microfibril orientation in the walls of elongating cells of collenchyma and subepidermal parenchyma can be best explained in terms of the concept of multi-net growth.