I just read the NYTimes' This Tree’s Leaves Look Soft and Inviting. Please Don’t Touch Them. which mentions the genus Dendrocnide and that Wikipedia section begins with:
Contact with the leaves or twigs causes the hollow, silica-tipped hairs to break off, penetrate the skin and inject the toxins.10 The hairs cause an extremely painful stinging sensation that could last from several hours to 1–2 days, recurring to a lessening degree for several months or more whenever the area is touched, exposed to water, or subjected to temperature change. The injured area becomes covered with small, red spots joining together to form a red, swollen welt. The hairs are also believed to be released to the air when the plant is cut or cleared in large areas. Workers without respiratory protection have reported sneezing, runny noses, mild nasal bleeding and throat irritation while cutting or clearing.
10Hurley, Marina. "'The worst kind of pain you can imagine' – what it's like to be stung by a stinging tree". The Conversation. Retrieved 2020-03-08.
and that article says:
The stinging hair structure is complex and consists of a tip, shaft and bulb composed of silica, calcium carbonate and calcium phosphate.
Reference 5 in Neurotoxic peptides from the venom of the giant Australian stinging tree (open access in Science) is Stinging hair morphology and wall biomineralization across five plant families: Conserved morphology versus divergent cell wall composition and this article contains a lot of information, for example following Figure 2:
Notable differences were found in the mineralization of stinging hairs. Three zones of the stinging hairs could be distinguished based on their mineralization patterns. The tip regions were generally strongly mineralized with highly concentrated silica or calcium phosphate, except for Cnidoscolus stinging trichomes, which were completely nonmineralized. The shafts were usually mineralized with calcium carbonate, but in some cases with calcium phosphate or silica besides calcium carbonate. Trichome bases showed more irregular mineralization, mainly with calcium carbonate and often additional silica deposits (Fig. 2K, L), or they were not yet completely mineralized (Fig. 2M).
This article primarily addresses structure and morphology, but I'm interested in how silica actually makes its way into a leaf structure. Unlike calcium and phosphorous, neither elemental silicon nor SiO2 readily dissolve in water, so I'd like to ask the following:
Question: Presumably it comes from minerals in the soil, but is it transported as particles of silica, or molecules of SiO2, or by other chemical forms containing silicon and reformed as silica on site, or something else?