Kilayim, Chapter 6, Mishnah 4. The Mishnah deals with a person who trained his vine over a fruit tree, and with the question of the status of the rest of the tree, the part that has no vine on it, regarding sowing seeds beneath it.
"HaMadleh es haGefen al Miktzas Ilan Ma'achal":
A person who trained his vine over the branches of a fruit tree - "Mutar Lehavi Zera Tachas HaMutar" - it is permitted to sow seeds beneath the remaining parts of the fruit tree that have no vine on them, because a person does not allow his fruit tree to become the permanent place of the vine. The expectation is that the owner of the vine will not let it spread over the rest of the tree, but will eventually remove it, and will not allow that to happen.
The vine spread over the place of sowing:
"Im Hilech HeChadash - Yachzirenu" - if the vine spread over the area in which the seed was sown, he must remove it from there. There is no prohibition after the fact, since the person's intention is that the matter is of no consequence to him, and therefore, as long as he removes the vine, it is permitted.
The incident in which Rabbi Yehoshua went to Rabbi Yishmael:
The Mishnah brings an incident - "Ma'aseh SheHalach Rabbi Yehoshua Etzel Rabbi Yishmael LiChfar Aziz" - Rabbi Yehoshua traveled to the place of Rabbi Yishmael, to Kfar Aziz, and there Rabbi Yishmael showed him two cases:
In Kfar Aziz: "VeHer'ahu Gefen Mudleh al Miktzas Te'einah" - a vine that had been trained over part of a fig tree, which is a fruit tree. Rabbi Yishmael asked: "Mah Ani Lehavi Zera Tachas HaMutar?" - what is the halachah, is it permitted to sow seed beneath the remainder of that fruit tree, the fig tree? Rabbi Yehoshua answered him: "Mutar" - permitted, in accordance with the ruling explained at the beginning of the Mishnah.
In Beis HaMaganya: From there Rabbi Yishmael brought him up to Beis HaMaganya, "VeHer'ahu Gefen SheHi Mudleh al Miktzas Korah VeSadan Shel Shikmah, UVo Koros Harbeh" - a vine that had been trained over the korah, that is, the branches, and over the sadan, which is the trunk, of a sycamore tree, and this tree had many koros ('korah' is the term used for the branches of a sycamore).
Rabbi Yehoshua ruled for him: "Tachas Korah Zu Asur, VeHaSh'ar Mutar" - beneath that branch over part of which the vine is trained it is forbidden to sow along its entire length, even in the section where the vine is not found; but the other branches are permitted. In a sycamore tree, each branch is viewed as a separate entity and an item in its own right, and therefore that particular branch is forbidden in its entirety, while beneath the other branches it is permitted to bring seed.
In summary: One who trains a vine over part of a fruit tree - it is permitted to sow beneath the rest of the tree, since a person does not designate his fruit tree as a permanent place for the vine; and if the vine spread over the place of the seeds, he should remove it, and there is no prohibition after the fact. In the incident in the Mishnah: with the fig tree, sowing beneath the remainder of the tree was permitted, and with the sycamore, sowing was forbidden beneath the entire branch over part of which the vine is trained, while beneath the other branches it is permitted, because each branch is considered a separate entity in its own right.