The uptake of monosilicic acid by crimson clover (Trifolium incarnatum L.) was investigated using solution cultures in which the level ranged from 0.4 to 60 ppm SiO2, and soils in which the level in solution ranged from 7 to 67 ppm SiO2. With increasing levels of silica in the external solution there were systematic increases in uptake, but the quantities of silica in the tops were always less than those which were theoretically carried to the roots in the mass flow of water. The silica content of the roots was higher than in the corresponding tops and seemed to be largely associated with the epidermis. These findings and the observation that the concentration of monosilicic acid in the xylem sap is lower than that in the external solution, are regarded as evidence that the plant excludes a proportion of the monosilicic acid from the transpiration stream. This exclusion is attributed to a barrier in the root through which monosilicic acid passes at a slower rate than water. The distribution of silica among the parts of the tops was unaffected by the level of monosilicic acid in the external solution and, in turn, by the quantity which entered the tops. The chemistry of silica and the pattern of its distribution in the tops suggest that the monosilicic acid which has moved across the root then moves concomitantly with water in the transpiration stream and that silica is deposited in greatest quantities in those parts from which water is lost in greatest quantities. © 1969 Martinus Nijhoff.
