Bikkurim, Chapter 1, Mishnah 2. In the first Mishnah of the tractate we learned that someone who has a share in a tree, but the tree draws its nourishment from ground that is not his, does not bring Bikkurim from it. Mishnah 2 comes to clarify the reason for this law: "Mei'eizeh ta'am eino meivi" - why are Bikkurim not brought when the fruits draw their nourishment from ground that does not belong to the one bringing them?
The reason for the exemption:
The Mishnah answers: "Mishum shene'emar reishis bikurei admascha" - the Torah stresses that the first of the fruits must come from the land of the one bringing them, "ad sheyihyu kol hagidulin me'admascha" - there is no obligation to bring Bikkurim unless all of the growth comes from his own land. Therefore, if part of the growth comes from elsewhere, from ground that is not his, as in the case of the previous Mishnah, the fruits are exempt from Bikkurim even though the partnership in the growth is only partial.
Those who do not bring for this same reason:
The Mishnah continues and lists additional categories of people who do not bring Bikkurim for this very reason:
"Aris" - a sharecropper, who works another person's field and gives the owner a certain percentage of what grows in the field.
"V'hechachir" - a tenant farmer, similar to a sharecropper, except that instead of giving a percentage of the crop he gives a fixed amount agreed upon in advance, a set quantity of fruit, regardless of how successful the orchard and its fruits turn out to be. This fixed amount is the rent he pays to the owner of the field.
"V'hasikarikon" - violent men who would drive owners off their land, threatening them and seizing their ground.
"V'hagazlan" - an ordinary robber, who takes over his fellow's land even without the use of violence.
"Ein mevi'in me'oso hata'am" - all four of these categories, the sharecropper, the tenant farmer, the sikarikon and the robber, do not bring Bikkurim from what grows on that land, "mishum shene'emar reishis bikurei admascha". In all of these cases, including the sharecropper and the tenant farmer, the land is not considered the land of the one working it, but rather the land of the owner or of the original owner, and therefore none of them can bring Bikkurim from it.
In summary: in this Mishnah we learned the reason for the exemption from Bikkurim on land that is not one's own - the verse "reishis bikurei admascha" teaches that there is no obligation unless all of the growth comes from the land of the one bringing them. For this reason the Mishnah lists the sharecropper, the tenant farmer, the sikarikon and the robber, whose land is not their own, and they too do not bring Bikkurim.