Mishnah 4 of this chapter takes up a general question: when a person plants produce that carries some restriction, what is the status of the new growth that comes up from those seeds? It opens with the subject of Terumah, which has occupied us throughout the chapter - the growths that come from Terumah, and even the generation after them, the 'grandchildren' of the Terumah kernels that were planted.
The Structure of the Mishnah:
The mishnah is packed with details, so we should first get clear on its structure. It has three parts:
First part: the status of the 'children' and the 'grandchildren' of Terumah seeds that were planted in the ground.
Second part: other categories of restricted produce, where what goes into the ground is restricted, and the question is what the status is of what comes out of it. This part speaks specifically about items whose seed disintegrates - the seed breaks down completely in the ground before the new growth sprouts - and regarding these it says that the new growths are halachically lenient and are not restricted. By contrast, when one plants an onion from these same categories the matter is restricted, even though the mishnah does not state this explicitly.
Third part: prohibitions that are lighter still, which do not continue to apply to what grows out of the ground, and this ruling is the same both for seeds whose seed disintegrates and for items whose seed does not disintegrate - such as an onion that is buried in the ground and does not rot, where the growth emerges from the onion itself, and even so what comes out of the ground is permitted. In these categories the restrictions are lost in the second generation, both with seeds and with onions.
The First Part: Growths of Terumah and Growths of Growths:
The mishnah opens with the principle that lies at the root of the entire chapter: "Giduley terumah - terumah" - growths of Terumah are Terumah. If one takes a seed of Terumah and plants it in the ground, what comes out of the ground is Terumah. This ruling, as was already explained in the last three mishnayos, is rabbinic.
However, "Giduley gidulin - chulin" - growths of growths are chulin. If one takes the growths that came out of the ground and plants them a second time, the third generation is not restricted as Terumah but is ordinary tevel. 'Chulin' here means as opposed to Terumah, but it is certainly not truly chulin, since it is obligated in the separation of tithes like any tevel.
The Second Part: Six Foods Whose Growths Are Chulin:
The wording of the mishnah is: "Aval tevel u'maaser rishon u'sfichey shevi'is u'terumas chutz la'aretz v'ha'meduma v'ha'bikkurim - giduleihen chulin" - but tevel, first tithe, aftergrowths of the Sabbatical year, Terumah from outside the Land, a mixture of Terumah with chulin, and first fruits - their growths are chulin. In these six, unlike Terumah whose growths are Terumah, what comes out of the ground is not restricted. This list applies only to seeds whose seed disintegrates, and not to onions - for if one planted onions of a Terumah mixture, for example, what comes out of the ground remains forbidden, as we have already seen in the tractate.
Tevel - food from which tithes have not been separated. Tevel may be eaten in a casual manner, as an informal nibble; but once its processing has been completed and it has been brought into the house, even casual eating becomes forbidden, and it may not be eaten until it has been tithed. Even so, if one plants a grain of tevel wheat whose processing was completed, what comes out of the ground is not restricted, and one may eat the wheat of the second generation casually without tithing it first.
First tithe - the first tithe, which is given to the Levi and is restricted to Levi'im alone. But if the Levi took it and planted it in the ground, what comes out of the ground is not considered tithe, and even someone who is not a Levi may eat it. It is worth noting, as the Bartenura points out, that in both of these cases there is still an element of Terumah embedded within: from tevel one must separate the great Terumah, and from first tithe the tithe of the tithe. But since this is only a small part of these foods, the Sages were lenient with them and did not apply to them the rule that growths of Terumah are Terumah.
Aftergrowths of the Sabbatical year - as you will recall from tractate Shevi'is, crops that are normally planted each year but grew on their own, such as wheat that sprouted from seeds scattered by the wind, are called sefichin. By Torah law they are permitted to be eaten, but the Sages forbade them, because people would act deceitfully and plant deliberately and then claim, "This is nothing but aftergrowth wheat." So sefichin are forbidden by rabbinic law. Even so, if one takes aftergrowth wheat and plants it, the growths that come from it are permitted and are chulin. Obviously we are not speaking of planting during the Sabbatical year itself, which is forbidden by the laws of Shemitah, but of keeping those aftergrowths until the eighth year and planting them then. The Sages did not see fit to restrict what comes out, since to begin with this is an unusual situation, and it is not their way to decree upon remote and uncommon scenarios.
Terumah from outside the Land - it has already been mentioned several times that even outside the Land of Israel there is a rabbinic obligation to separate Terumah in the neighboring lands: Ammon and Moab (today's Jordan), Suria (Lebanon and Syria), Egypt and Babylonia (Iraq) - in all these regions produce is obligated in separation by rabbinic law, at least in the era of the Mishnah. And although the rule is that growths of Terumah are Terumah, here, since the obligation is only rabbinic, the Sages were lenient and did not say that what comes out is Terumah. The Bartenura offers a different reason: just as aftergrowths of the Sabbatical year are uncommon, so too Terumah from outside the Land is uncommon. At first glance this is difficult, for someone living in Babylonia who must tithe everything - Terumah is not at all uncommon for him, but is as common as in the Land of Israel. Many therefore explain that this entire discussion takes place only in the Land of Israel, and there it is uncommon to find Terumah that was brought in from outside the Land, and so no decree was made about it. According to this explanation, the entire framework of growths of Terumah being Terumah applies only in the Land of Israel, and with Terumah that arrived from outside the Land they were lenient both because it is only rabbinic and because it is uncommon.
A mixture of Terumah with chulin - a mixture of chulin and Terumah. As we saw earlier in the tractate, since such a mixture is only a rabbinic matter, the stringencies of the Terumah mixed into it still apply to it; but by Torah law, since the majority of it is chulin, the Terumah is nullified in the majority and it is chulin. For this reason we are lenient here, and even though it contains a small amount of Terumah, since it is nullified we do not apply the stringency to it.
First fruits - the first fruits in the Land of Israel, from the seven species for which the Land is praised: wheat and barley, olives, dates, grapes, figs and pomegranates. A farmer who sees the first pomegranates budding on his tree ties a red string around them, and when they ripen he takes them and brings them up to the Beis HaMikdash to wave them and give them to the kohen, and the kohen eats them like Terumah. As was stated in the introduction to the tractate, there are four things whose status is like Terumah: the great Terumah, the tithe of the tithe, challah from the dough, and first fruits. Although first fruits resemble Terumah, we are lenient and say that if one plants a pomegranate of first fruits or sows wheat of first fruits, what grows from them is unrestricted. And the Bartenura's reason is that it is simply not found in the Land of Israel that people occupy themselves with planting items of first fruits, and therefore the Sages did not restrict the second generation of their offspring.
The Third Part: Growths of Consecrated Property and of Second Tithe:
This part deals with two categories of forbidden foods whose second generation coming out of the ground is not forbidden, whether they were seeds whose seed disintegrates or onions: "Giduley hekdesh u'maaser sheni - chulin" - growths of consecrated property and of second tithe are chulin.
Hekdesh (property consecrated to the Temple): Someone who had a grain of wheat and consecrated it for the upkeep of the Temple, so that the Temple would have an asset from which money could be raised - if those seeds were sown in the ground, whatever grows from the ground is permitted. Maaser sheni (second tithe): the tithe that a person is meant to eat in Jerusalem - if he sowed it in the ground, what grows from the ground is permitted, and may even be eaten outside Jerusalem.
The reason for the decree that "produce of Terumah is Terumah":
This is the place to complete the explanation mentioned in the opening mishnayos of the chapter: what is the logic underlying the rabbinic law that "produce of Terumah is Terumah". Understanding this will also help us understand why it does not apply at all to produce grown from hekdesh and maaser sheni.
A well known Gemara in the second chapter of Maseches Shabbos relates that Beis Shammai and Beis Hillel gathered in the upper story of a certain person's house (the details of the episode are disputed and are not relevant to our discussion). On that occasion there happened to be a quorum equivalent to a Sanhedrin able to rule on halachah - a once in a lifetime opportunity - except that the majority of those present were from Beis Shammai. Beis Shammai seized the opportunity and enacted eighteen rabbinic decrees, those very matters that had long been dear to them and that they had never managed to pass, since the majority of the Sanhedrin were from Beis Hillel. These are the famous "eighteen matters".
On daf 17 in Maseches Shabbos one of those eighteen matters is listed: produce of Terumah is Terumah - one who sows Terumah, what grows from the ground is Terumah, and this is the subject of our entire chapter. Most of the eighteen matters deal with questions of tumah and taharah, and this enactment is among them.
And this is the reasoning of Beis Shammai: a kohen who received Terumah, such as kernels of wheat, which he is meant to eat - if the wheat became tamei, he can no longer eat it, and he has no choice but to destroy it and burn it. But he may well have a "plan B": to take the kernels of wheat, sow them in the ground, and grow an entirely new crop that would be a clean slate - ordinary produce with none of the restrictions of Terumah.
The problem is that Beis Shammai were concerned that if the kohen were to keep tamei kernels of wheat in his house until planting season arrived - and one sows only once a year, in the planting season close to Tishrei - he might come to eat them by mistake. The houses were small, and wheat is not merely seed but also food, and keeping forbidden food in the kitchen for an entire year is a disaster waiting to happen. Said Beis Shammai: if we remove the financial incentive to hold on to the wheat, he will not keep it but will get rid of it, and in this way the inadvertent eating of tamei Terumah wheat will be prevented.
What did they do? They ruled that if one sows his Terumah, what grows from the ground remains Terumah. This undermined the entire financial benefit of holding on to the kernels of wheat, since the price of wheat that is Terumah is only a tiny fraction of its price as ordinary produce: only kohanim may eat it, and even among them only a kohen who is tahor. No sensible kohen would use his field to grow Terumah wheat, which is economically inferior. That was the goal, and that is the motivation behind the entire chapter.
The enactment passed when Beis Shammai found themselves in the majority, and Beis Hillel saw fit to leave it in place and did not annul it when the Sanhedrin convened a second time and they were in the majority. For this reason those eighteen enactments remained binding in practice, and this is one of them.
Let us return to the mishnah:
In light of this, the final section - produce grown from hekdesh and maaser sheni - is understandable. Here there is no reason in the first place for a person to sow these seeds instead of giving them to the Temple or eating them in Jerusalem. First, no one ever obligated him to consecrate them. Second, if he wanted those particular seeds for some reason, he could have redeemed them and bought them back. So too with maaser sheni: if he plants it in the ground he does indeed avoid going up to Jerusalem, but in any case he could have redeemed it onto coins and rid himself of the problem. Since this is so, the Sages saw no place to decree regarding their produce, and the mishnah ruled that produce grown from hekdesh and maaser sheni is ordinary produce.