Orlah, Chapter 1, Mishnah 4. This Mishnah continues the subject of the previous Mishnah, and it too deals with a tree that was uprooted from its place.
"Ilan shene'ekar v'nishtayer bo shoresh - patur":
A fruit tree whose years of orlah have already passed, so that its fruit is permitted to be eaten, and which was afterward uprooted from its place - if a root remained attached to the ground, the tree is exempt from the laws of orlah, and one does not begin counting the years of orlah anew.
"V'chamah yehei hashoresh?" - And how large must the root be?
That is, what is the required measure of the root that remained? On this Rabban Gamliel says in the name of Rabbi Eliezer ben Yehudah of Bartota: "K'machat shel miton" - the root must be as thick as the needle that weavers use in the process of weaving a garment, which is a relatively large needle. With a root of this size the tree is considered attached to the ground, and the count of the orlah years does not begin anew.
In summary: in this Mishnah we learned that a tree that was uprooted and retained a root attached to the ground is exempt from the laws of orlah, and its orlah years are not counted anew, and the measure of that root is "k'machat shel miton", as stated by Rabban Gamliel in the name of Rabbi Eliezer ben Yehudah of Bartota.