Kilayim, Chapter 1, Mishnah 7. This mishnah opens the discussion of grafting - taking a branch from one tree and attaching it to another tree. As we will see, the discussion extends to vegetables as well.
Grafting a tree onto a tree:
One may not graft a branch of a fruit tree onto another fruit tree. This prohibition is not limited to two fruit trees, but also includes:
A branch of a fruit tree onto a barren tree - a tree that does not bear fruit.
A branch of a barren tree onto a fruit tree.
All of these cases involve two different species. In contrast, it is permitted to graft two kinds of barren trees onto one another, since with regard to kilayim all barren trees are considered a single species, and there is no prohibition of kilayim in grafting them at all.
Additional grafts that were prohibited:
The mishnah goes on to list which things may not be grafted together:
"yarak b'yarak" - one may not graft one kind of vegetable onto another kind of vegetable.
"v'lo ilan b'yarak" - one may not graft a branch of a tree onto a vegetable.
"v'lo yarak b'ilan" - one may not graft a branch of a vegetable onto a tree.
The view of Rabbi Yehudah:
"Rabbi Yehudah matir yarak b'ilan" - in his opinion it is permitted to graft a vegetable onto a tree, because Rabbi Yehudah holds that the prohibition of kilayim applies only where a new species may grow out of the graft. With two kinds of fruit trees, joining them together carries within it the potential that a new species will grow as a result of that graft.
A vegetable and a tree, however, are too far removed from one another to produce something new, and no additional species will grow from that joining. And something from which nothing grows as a result of the joining is, in Rabbi Yehudah's view, not prohibited as kilayim.
In summary: in this mishnah we learned the prohibition of grafting - a fruit tree onto a fruit tree, a fruit tree onto a barren tree and a barren tree onto a fruit tree, while grafting two kinds of barren trees is permitted, since they are all considered a single species. Likewise prohibited are grafting a vegetable onto a vegetable, a tree onto a vegetable and a vegetable onto a tree, and Rabbi Yehudah permits a vegetable onto a tree because their grafting does not produce a new species.