Chapter 7, Mishnah 8. The Mishnah opens with the law of one who consecrates his vineyard, and then moves on to discuss the parameters of shichechah (forgotten produce) in a vineyard.
One Who Consecrates His Vineyard:
"HaMakdish karmo ad shelo nod'u bo ha'olelos - ein ha'olelos la'aniyim" - if a person consecrated his vineyard before it was possible to identify which grapes are olelos (defective clusters left for the poor), since they had not yet grown, then when the olelos do grow they do not belong to the poor, because the consecration preceded them and took effect on them.
"Mishenod'u bo ha'olelos - ha'olelos la'aniyim" - but if he consecrated his vineyard after the olelos were already recognizable, they belong to the poor and not to hekdesh (the Temple treasury).
The reason: at the moment the owner consecrates his vineyard, if the olelos are already recognizable in it, those olelos are not his but the property of the poor, and a person cannot consecrate something that is not his. Therefore the olelos go to the poor and not to hekdesh.
The Opinion of Rabbi Yosi:
Rabbi Yosi adds: "Yitnu s'char gidulah l'hekdesh" - they must pay hekdesh for the growth. Even though the olelos belong to the poor, since their grapes are nourished by the rest of the vineyard, which belongs to hekdesh, the poor are required to pay hekdesh the value of that growth, for they drew their nourishment from the property of hekdesh.
The Gemara disagrees with this reasoning and holds that since the vineyard was already obligated in olelos before the consecration took effect upon it, the olelos are entitled to draw nourishment from hekdesh with no obligation toward it whatsoever.
Shichechah on a Trellis:
From here the Mishnah moves to the laws of shichechah in a vineyard: "Eizohi shichechah be'aris? Kol she'eino yachol lifshot es yado velitol" - what is shichechah on an aris? Anything that he cannot reach out his hand and take. In a vineyard growing on an aris - a vine trained over poles - we must determine the point of no return during the harvest, from which point on the vine is considered forgotten. The Mishnah establishes: whenever the harvester can no longer stretch out his hand behind him and take the grapes, he has passed the point at which he could have returned to them, and from now on it is considered shichechah.
The Yerushalmi gives a reason for this: it is the way of a vineyard owner to go back and check as long as he is still able to reach the spot; but once he has passed that point, he no longer checks, since he has given up on it, and therefore it becomes shichechah.
Shichechah at One's Feet:
This ruling was stated regarding an aris, a vine growing on poles. But what is the law concerning vines that lie along the ground, right beside the feet of the harvester? On this the Mishnah says: "U'varegel - mishe'avar heimenah" - and at one's feet, once he has passed it. From the moment he has moved past the vine, it is immediately considered shichechah.