So that leaves two questions. First, what is Thunberg’s plant? It’s actually Citrullus amarus, the citron melon. Its alternative names, preserving melon or jam melon, reflect the fact that it’s rarely eaten raw, but has (apparently) been used for making jam for centuries. And if you’ve never heard of citron melon, you’re not alone, I hadn’t either… nor has anyone else as far as I can tell. My New Oxford Book of Food Plants mentions it not, there’s no supplier in the RHS Plant Finder, and it may not even be in cultivation in the UK, so news of anyone who has tried growing it, or eaten the jam, would be welcome.