The ability to metabolize glass [silicon dioxide] appears to have evolved independently in two separate groupings of plants - the algal diatoms, and the land equisetums [horse-tails, or scouring-rushes, as modern extant species are called]. Diatoms exist today little-changed from their ancient ancestors, and modern silicon-dioxide shells also exist in the fossil record from hundreds of millions of years ago, showing a great evolutionary niche they have occupied. Likewise, modern horsetails look quite similar to ancient fossils, though fewer species exist today than anciently. Glass is generally considered 'insoluble' in water, though this is evidently not quite true, as silicon dioxide must be in solution in order for it to be metabolized. So, what is the metabolic pathway that is utilized in these two diverse groupings of plants? In other words, what is the transport mechanism for a dissolved SiO2 molecule from solution to the external shell of a diatom? of a horsetail?
