Kilayim, Chapter 6, Mishnah 5. In the two previous mishnayos we examined the distinction between a barren tree and a fruit tree - between a tree that does not bear fruit and one that does. A barren tree with a vine growing on it has the status of the poles of an aris (trellis), and the halachah is that we assume the tree will eventually be covered entirely by the vine, and therefore one may not sow beneath any part of it. Beneath a fruit tree, by contrast, one may sow under its other parts.
Defining a barren tree:
Our mishnah takes up the question of what is considered a barren tree. The mishnah asks: "Eizehu ilan serak?" - which tree is considered barren, that is, a tree that does not bear fruit; and it answers: "Kol she'eino oseh peiros" - any tree that produces no fruit at all. This definition seems straightforward, but the mishnah brings two additional opinions:
Rabbi Meir: "Hakol ilan serak" - all trees, even those we ordinarily call fruit trees, are considered barren trees. They are not so important and valuable that a person would refrain from letting a vine grow on them. "Chutz min hazeisim umin hate'einah" - only the olive tree and the fig tree are valuable enough for us to say that a person would not let a vine grow on them, and therefore one may sow beneath all other trees.
Rabbi Yosi: "Kol she'ein kemoso not'in sados sheleimos" - any type of tree from which people do not plant entire fields, meaning entire orchards, but only individual trees, is not considered a fruit tree but a barren tree.
In summary: this mishnah records a dispute about the definition of a barren tree: according to the anonymous opinion of the mishnah, any tree that produces no fruit; according to Rabbi Meir, all trees except the olive and the fig; and according to Rabbi Yosi, any tree from which people do not plant entire fields.