Kilayim, Chapter 1, Mishnah 3. Like the first two mishnayos of the tractate, this mishnah also deals with pairs of items that appear to be different species, but in fact, as far as the laws of kilayim are concerned, they are considered a single species and may be planted together.
The three pairs in the mishnah:
"HaLefes v'haNafutz" - the lefes is a root vegetable (turnip), and the nafutz is a type of radish whose leaves resemble the leaves of the turnip. The two are considered one species.
"HaKruv v'haTerovtor" - the kruv is the cabbage familiar to us, and the terovtor is a wild variety of cabbage (and some wish to say that it is cauliflower). These two as well are considered the same species.
"HaTeradin v'haLu'onin" - these are two types of beets, and they are not kilayim with one another.
Each of these pairs, each pair on its own, is considered a single species and is not subject to the laws of kilayim.
"Hosif Rabbi Akiva":
Rabbi Akiva added three more pairs:
"HaShum v'haShumanis" - garlic and wild garlic.
"HaBatzal v'haBetzaltzul" - onion and wild onion.
"V'haTurmus u'Falaslos" - the turmus is a type of legume, a species of bean (lupine), and falaslos is wild lupine.
"Einam kilayim zeh bazeh" - the wild versions of each of these items are considered the same species as the item itself, and therefore they are not kilayim with one another.
In summary: This mishnah lists pairs that appear to be different species but are treated as a single species: the turnip and the nafutz, the cabbage and the terovtor, beets and lu'onin, and with Rabbi Akiva's addition - garlic and wild garlic, onion and wild onion, and lupine and wild lupine. In each of them the wild variety is considered the same species as the cultivated one, and they are not kilayim with one another.