Silicon (Si) has diverse roles in plants including in stress tolerance and protecting plants against pathogens and herbivores. Silicon is abundant in soil and in many plants, particularly in the horsetail Equisetum, and liverworts. Its biology is less well understand than that of other minerals, so this study that looks at silicon uptake in Equisetum is quite interesting.
Here is a review article on the topic:
Si uptake in plants: Trends Plant Sci 2006 http://www.cell.com/trends/plant-science//retrieve/pii/S1360138506001634
Thanks to Tom for the photo:http://www.flickr.com/photos/clearwood/7187234331/in/photostream/