Nodal roots of mature, soil-grown specimens of Sorghum bicolor (L.) Moench. were investigated for their Si content by means of the electron-probe microanalyser and the scanning electron microscope. No consistent accumulation of Si occurred other than in the endodermis. Indication of a lower Si content in the old as compared to the younger nodal roots was obtained. The solid silica deposits occurred as domeshaped silica aggregates confined to the inner tangential wall. Comparisons with the similar deposits of young seminal roots are made. It is argued that the endodermal deposits represent a specialized aspect of silicification and contrasting hypotheses to account for it are proposed, one involving physico-chemical factors, the other, protoplasmic control.
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