Kilayim, Chapter 2, Mishnah 11. This mishnah deals with a case in which two species of seeds are planted with the proper separation between them, as required, but their plants lean over and become intertwined one above the other. The question is whether the separation at the place of planting is sufficient, or whether the leaning above also creates a prohibition.
The text of the mishnah:
"Tevuah notah al gabei tevuah, v'yarak al gabei yarak, tevuah al gabei yarak, yarak al gabei tevuah - hakol mutar, chutz mid'laas Yevanis" - grain leaning over grain, vegetable over vegetable, grain over vegetable, vegetable over grain - it is all permitted, except for the Greek gourd.
The four cases in the mishnah:
"Tevuah notah al gabei tevuah" - two different species of grain, planted with the proper separation where they grow, and one of them leans over the other.
"V'yarak al gabei yarak" - one species of vegetable leaning over another species of vegetable.
"Tevuah al gabei yarak" - grain leaning over a vegetable.
"Yarak al gabei tevuah" - a vegetable hanging over grain.
In all of these cases "hakol mutar" - it is all permitted, because what matters is the separation at the place where the seeds are attached to the ground. It makes no difference that the plants hang and lean one over the other.
Except for the Greek gourd:
The exception is the Greek gourd, whose leaves were very broad, and they would become mixed together with the other species near them to the point that it looked like kilayim. Therefore, if the Greek gourd leans over grain or over another vegetable, it must be uprooted.
The view of Rabbi Meir:
Rabbi Meir says: "Af kishus u'pol haMitzri" - also the cucumber and the Egyptian bean. That is, these plants present the very same problem, and they too are included in the exception. But he continues: "V'ro'eh ani es divreihen midivrai" - and I regard their words as preferable to my own. Even though he possessed a tradition extending the prohibition to the cucumber and the Egyptian bean in addition to the Greek gourd, he prefers the position of the Sages, who limit the exception to the Greek gourd alone and nothing more.
In summary: One species leaning over another does not create a prohibition, since the required separation is at the point of attachment to the ground, and therefore "hakol mutar" - it is all permitted. The exception is the Greek gourd, whose broad leaves become mixed with the neighboring species and appear to be kilayim, and the ruling is that it must be uprooted. Rabbi Meir adds the cucumber and the Egyptian bean on the strength of his tradition, but he himself rules "v'ro'eh ani es divreihen midivrai" - in accordance with the view of the Sages, that the only exception is the Greek gourd.