Kilayim, Chapter One, Mishnah Eight
This mishnah forbids certain practices because they are considered grafting or close enough to it. These examples demonstrate cases in which a person might have grafted a vegetable onto a tree, a tree onto a vegetable or one vegetable onto another.
I should note that vegetable in rabbinic literature means anything that is not a tree, grain or bean. Thus herbs are vegetables.
1) They may not plant vegetables in a trunk of a sycamore tree.
2) They may not graft rue on white cassia, since that is [grafting] a vegetable on a tree.
3) They may not plant a young fig-shoot in sea squill so that it might provide shade for it.
4) They may not insert a vine branch into a melon, in order that the latter might shoot its moisture to the former, since that is [grafting] a tree on a vegetable.
5) They may not place gourd seed into anchusa for the purpose of preserving it (the gourd seed), since that is [grafting] a vegetable on another vegetable.
Section one: It seems that people may have planted vegetables in the trunks of sycamore trees that had been cut down. Assumedly, the vegetables would be able to derive nutrients from the decomposing wood. Alas, while this might be a good idea, it is prohibited because of kilayim.
Section two: A rue is a shrub, which falls in the vegetable family (according to rabbinic classification, of course) and a white cassia is a tree. Thus grafting the two together is prohibited.
Section three: The sea squill is a bulbous plant whose roots grow deep. It seems that planting the young fig in the sea squill would help cool it by providing shade. However, this is kilayim and is therefore prohibited.
Section four: A vine is considered a tree and a melon is a vegetable. Hence it is forbidden to stick the vine into the melon to use it as a source of moisture.
Section five: It seems that the moisture of the anchusa plant could be used to preserve gourd seed. Our mishnah teaches that although the intent here is not to actually graft one plant on another but just to preserve them, it is still forbidden due to kilayim, in this case one vegetable with another.