Kilayim, Chapter 3, Mishnah 5. In the previous mishnah we learned that the leaves of cucumbers, hops, gourds and pumpkins are very long and become intertwined with one another. Even so, as our mishnah describes, there is a situation in which one may plant them in complete proximity to each other: when a person provides a heker, a noticeable sign that makes it clear that they are not being planted together, despite how close they are.
The wording of the mishnah:
"Notei'a adam kishus u'dela'as l'soch guma achas" - one may plant a cucumber and a gourd in a single hole, with no distance at all between them.
"Uvilvad shetehei zo notah l'tzad zeh v'zo notah l'tzad zeh" - the two plants are directed toward different sides.
"V'noteh se'ar shel zo l'chan u'se'ar shel zo l'chan" - the mishnah goes further and adds that the leaves of the one plant are also turned in one direction, and the leaves of the other plant in the other direction.
When the matter is plain to the eye, and the plants are pushed toward different sides of the hollow, the hole in which they are planted, it is permitted.
The reason for the leniency:
This leniency is possible, the mishnah explains, because "shekol mah she'asu chachamim v'gazru alav mipnei mar'is ha'ayin" - everything that the Sages forbade in matters of kilayim is only a decree on account of the way the thing appears. Therefore, when a person provides a heker, a sign that makes the matter noticeable, such as turning the leaves in different directions, and it becomes clear that the planting is not being done with the intention of mixing the plants together, it is permitted.
In summary: in this mishnah we learned that one may plant a cucumber and a gourd in a single hole, provided that each one leans toward a different side and its leaves are turned in a different direction, since the rabbinic prohibitions of kilayim were decreed on account of how the matter appears, and the distinguishing sign removes that appearance and permits the planting.