Maasros, Chapter 4, Mishnah 5. This Mishnah too deals with eating kinds of produce that were taken before the work on them was completed, and with the possibility that gathering even a small quantity counts as a miniature completion of work that obligates the produce in maaser.
"Mekalef se'orim" - barley eaten away from the threshing floor:
The Mishnah opens with barley, and it is speaking of barley that was taken from the threshing floor and is eaten elsewhere. At the threshing floor itself, eating raw barley is not really a consideration, and in any case it would be no more than a casual snack. The question is at what point the eating becomes an official meal:
"Mekalef achas achas ve'ochel" - one who peels the husk off the barley and eats one at a time. Some explain this as one kernel at a time, and the Tiferes Yisrael explains it as one whole ear at a time. Since he is not accumulating any quantity, this is casual eating.
"Kilef venasan lesoch yado" - one who peels the barley kernels and gathers a pile of them in the palm of his hand. Such a pile is like a small quantity that has undergone smoothing over, and it is therefore obligated in maaser.
"Hamolel melilos shel chittim":
The word "molel" means rubbing between the hands. When a person takes an ear of wheat and gently rubs it in his palm, the hard chaff comes loose and falls away, and what remains are the wheat kernels - dry and uncooked, separated but still mixed with bits of husk. (For someone who has never seen this: think of peanuts mixed together with that soft reddish skin.) According to the Bartenura, we are dealing with roasted kernels, and after the rubbing there is a mixture in his hand of edible kernels together with crumbs of chaff. Two scenarios are presented:
"Menape'ach miyad leyad" - he tosses the kernels from one hand to the other, which is a form of winnowing: the chaff and husks fly off and fall to the ground, and the heavier kernels are separated out. This is casual eating.
"Venasan lesoch cheiko" - he separated the kernels from the chaff and from the small husks and gathered a pile of them in the folds of his garment, intending to eat the pile as it was collected. He now has a miniature harvest before him, and the wheat is obligated in maaser before he may eat it.
It is worth noting a difficulty in the words of the Bartenura, who learns from the Rambam that we are dealing with roasted wheat: seemingly, roasting by fire in itself establishes the obligation in maaser, and if so it is hard to understand why any roasted wheat would not be obligated in any event. For this reason we adopt a different and simpler reading, that this case is like "mekalef se'orim": he peels off the husks and crushes the crumbs of wheat in his hands, with no roasting at all. According to this it is understandable why one could eat them one at a time without gathering a separate pile in the fold of his garment.
Kusbar - seed and vegetable:
The kusbar plant yields two separate things: the seeds - these are the coriander seeds, which are its primary use and the usual reason it was planted in the times of the Mishnah; and the leaves - this is cilantro. In the days of Chazal people were not eager to eat the leafy greens, and therefore one who plants kusbar without specifying has in mind the seeds alone, and the leaves are not significant to him as a fruit. From here comes the halachah:
He planted it for the seed: the green part, the cilantro, is entirely exempt from maasros.
He planted it for the vegetable: he intends to eat the leaves as cilantro, and therefore he must tithe the leaves as well before eating them - and of course the seeds too, since they are the plant's primary produce.
It emerges that there is no way to sidestep the obligation to tithe the coriander seeds, since that is the plant's normal produce; whereas the obligation to tithe the leaves exists only when they were planted specifically for the cilantro.
The dispute between Rabbi Eliezer and the Sages regarding dill:
Rabbi Eliezer says: "HaShevet mis'aser zera v'yarak v'zirin" - every edible part of the dill plant is obligated in maaser:
Zera - the dill seeds, the kind found in a jar of pickles.
Yarak - the dill leaves, which are also used in pickles, in soup and the like.
Zirin - the shoots or stalks on which these grow.
According to him, these are all the ordinary edible parts, and it is for their sake that dill is planted. Since that is so, once a person plants the crop, all of its parts automatically become obligated in maaser before they may be eaten.
The Sages disagree: "Ein mitztaref zera v'yarak ela hashechalayim v'hagargir". In their view, someone who plants such crops generally has the seeds in mind and is not particular about the leafy part, and that was indeed the situation in the days of the Mishnah. Therefore, in any plant whose seeds are edible, the intent is for the seeds, and the green part does not require maaser unless he specifically intended to eat the greens, just as we learned regarding coriander. Dill itself is included in this rule: in disagreeing with Rabbi Eliezer, the Sages hold that the green part does not require maaser, since under ordinary circumstances the dill seed, and not its leaves, is the primary produce.
There are only two exceptions, and dill is not among them:
Shechalayim - a leafy green whose precise identity is unclear; some translate it as rocket.
Gargir - apparently arugula.
These two greens are so tasty that anyone who plants them automatically intends to eat the leaves as well. Therefore both the greens and the seeds are obligated in maaser, regardless of the unstated intent at the time of planting. With every other type of plant the intent is for the seeds, and the leaves are exempt from maaser unless the leaves in particular were on his mind when he planted them.
In summary: in this Mishnah we learned that one who peels barley and eats the kernels one by one, and likewise one who rubs out kernels and blows them from hand to hand, is eating a casual meal; but one who gathers a pile in his palm or in his lap has created a miniature form of completion of work and has become obligated in maaser before eating. We also learned the law of coriander, whose leaves are exempt unless it was planted for the sake of the greens, and the dispute between Rabbi Eliezer and the Sages regarding dill - whether all of its parts are obligated in maaser, or whether the only exceptions are shechalayim and gargir.