Which part of a plant is caffeine made from?

Caffeine can be derived from the coffee bean, from the tea leaf, from the guarana seed, and from the kola nut, as well as many other plant sources. Some plants have it mostly in their leaves, where it is used as a natural defense to paralyze insects, and some use it in the fruit and seeds, and therefore these portions of the plants are the ones where the caffeine concentration will be highest.