Oktzim, Chapter Two, Mishnah Seven
1) The [outer] leaves of vegetables: if they are green they join together, but if they have whitened they not join together.
2) Rabbi Elazar bar Zadok says: the white leaves of cabbage join together because they are edible.
3) So also those of lettuces, because they preserve the edible part.
Section one: If the outer leaves are green, they are still edible and therefore they are included as part of the vegetable. Once they have whitened, they are no longer part and are not included.
Section two: White outer leaves of cabbage are still edible so they join together.
Section three: While the outer leaves of lettuce may not be eaten, they join together because they protect the inner part.