This is a review paper only in the sense that much pertinent biochemical literature is reviewed for its bearing on the thesis that accumulator plants are important geologic factors, a proposal first made in 1958 (Lovering, 1958). I am not a "biogeochemist", and the ideas presented here must stand on their own feet —but at least they have the merit of being uninfluenced by current dogma in the fields discussed. I am, however, deeply indebted to many friends in these fields for suggestions, especially as to the vast and unfamiliar literature; and foremost among these helpful guides, I am happy to acknowledge the indispensable aid given to me by H.W. Lakin, soil scientist, geochemist, and currently chief chemist of the geochemical exploration section of the U.S. Geological Survey. Although I stress evidence for the conclusion that vast quantities of SiO2 are pumped out of the ground by silica-accumulator plants, which sooner or later release the silica in a form very vulnerable to physical transport by runoff or wind erosion, it should be made clear that we cannot yet appraise the value of this concept until much field work is done. I am not yet persuaded that erosive agents remove the major part of the vegetal silica in the tropics—or elsewhere—but I am convinced that this possibility must be given a serious study. At present we have absolutely no quantitative data on the relative amounts of silica taken up by various plant communities, nor any factual information as to the proportions of such silica recycled by plants, dissolved, and moved down into ground water, swept off the surface by erosive agents, or left behind as an insoluble residue from decaying plant litter. And I believe we should set about getting this information.