In an old school technique of duplicate verification, contracts were written in duplicate or triplicate on the same piece of paper, then cut apart with an arbitrary jagged or wavy pattern. If the two parts fit, you knew they were from the same original. From the toothed pattern, the contracts were called "indentures", hence "indentured servants."
I'd always assumed "plantations" were so-called because they were places where things were planted. And while the OED has citations for that sense dating as far back as 1569, its use to describe the English plantations in Ireland and North America derives from their being an implantation of settlers.