Pe'ah, Chapter 4, Mishnah 11. This mishnah deals with the question of how to treat grain that is found when there is a doubt that it may have come from leket, the gleanings that fell during the harvest, and therefore belongs to the poor.
"Chorei hanemalim shebetoch hakamah":
The mishnah opens: "Chorei hanemalim shebetoch hakamah - harei hen shel ba'al habayis" - ant holes that are within the standing grain, their contents belong to the owner. The case is ant holes in which kernels of grain were found. There are two ways to understand the phrase "within the standing grain":
In terms of place: the holes are located in the area of the standing grain, in a spot where the harvesting has not yet taken place.
In terms of time: this is while the grain is still standing, before any harvesting has taken place at all.
In both understandings the kernels belong to the owner, since they cannot have come from leket. In a place where the harvesting has not yet happened, we assume that the kernels fell just now, or that the ants gathered them in some manner, and they do not carry the law of leket.
"Shele'achar hakotzrim":
However, when we are dealing with a time after the harvesting has already been done, or with an area where the harvesters have already cut the grain, the mishnah distinguishes between the kernels inside the holes:
"Chittin ha'elyonim" - the upper wheat kernels go to the poor, since they may well have come from the leket.
"Vehatachtonim shel ba'al habayis" - the lower ones belong to the owner, and the law of leket does not apply to them.
The explanation of the Yerushalmi - what are the upper ones and what are the lower ones:
The upper ones are the white ones - kernels that look completely ripe. Since they appear to have been cut at the time of the harvest, and leket always comes from ripe kernels, they belong to the poor.
The lower ones are the green ones - kernels that have not yet ripened, which therefore fell before the time of the harvest, and so they belong to the owner.
The reason the mishnah used the terms "upper" and "lower" is that the green ones are usually found at the bottom of the hole, while the white ones, which have ripened fully, are found in its upper part.
The view of Rabbi Meir:
Rabbi Meir disagrees and says: "Hakol la'aniyim, shesafek leket leket" - everything found in the ant holes belongs to the poor, since anything about which there is a doubt or a possibility that it is leket is presumed to be leket. Some derive this from the verse "le'ani velager ta'azov" - "for the poor and for the stranger you shall leave it," where the extra expression of leaving teaches that even in a case of doubt one must give to the poor.
If we are speaking about green kernels, why is there any doubt at all?
Seemingly they certainly belong to the owner, since they come from grain that had not ripened. The Yerushalmi gives the reason: "Ee efshar legoren shetetzei belo yerukim" - it is impossible for a threshing floor to come out without green kernels. There is no pile of grain and no harvest of grain that does not contain a few kernels that have not ripened. Even at the time of the harvest, although the kernels are generally white and fully ripe, there are inevitably some among them that have not ripened. Therefore, even the green kernels found in the ant holes may have come from kernels that fell during the harvest, and they truly belong to the poor. And since Rabbi Meir holds by the rule of "a doubtful leket is leket," one must be stringent and assume that these too belong to the poor.
In summary: in this mishnah we learned that ant holes found within the standing grain - their grain belongs to the owner, and the law of leket does not apply to it; whereas after the harvesters have passed, the mishnah distinguished between the upper kernels, the white and ripe ones, which belong to the poor, and the lower ones, the green ones that had not ripened, which belong to the owner. Rabbi Meir disagrees and applies the rule of "a doubtful leket is leket," and since there is no threshing floor that comes out without green kernels, even the green kernels may have come from the harvest - and therefore everything goes to the poor.