Kilayim, Chapter Three, Mishnah Five
This mishnah teaches that one can sometimes plant two different species in one hole, as long as it doesnt look like kilayim.
1) One may plant a cucumber and a gourd in one hole, as long as this [species] inclines in one direction, and the other [species] in the opposite direction.
2) And he should tip the leaves of one [species] one way, and the other the opposite way, since all that the sages prohibited [in matters of kilayim] they only decreed because of appearance.
Section one: It is permitted to plant a cucumber and a gourd plant in one hole, as long as the leaves are inclining in opposite directions, so that it doesnt look like kilayim.
Section two: This entire section is missing from good manuscripts of the mishnah and is clearly a later addition, meant to explain section one. The main point is that from the Torah kilayim are only prohibited if the two species are actually mixed together. The rabbis added that if the two species look like they are mixed together, then it is prohibited derabanan. In the case of the cucumber and gourd whose leaves incline in opposite directions, since it doesnt look like the two are intertwined, it is not prohibited at all.