Shevi'is, Chapter 9, Mishnah 1. This mishnah lists a number of wild vegetables that grow on their own, which are generally considered ownerless and free for the taking. For that reason certain leniencies were applied to them during the shemittah year. We will go through them one by one, even though most of these items, and even their names, are unfamiliar to us today.
The vegetables listed in the mishnah:
"HaPeigam" - a type of vegetable that grows in the mountainous regions of Eretz Yisrael.
"V'hayarbuzin hashotim" - a type of herb. 'Shotim' means that it grows wild, as opposed to being cultivated in a field.
"V'hachalaglogos" - another type of herb, known as purslane.
"V'chusbar shebeharim" - coriander that grows in the mountains.
"V'hakarpas shebanharos" - a species close to celery or parsley, which grows alongside rivers.
"V'hagargeir shel afar" - arugula that grows in the field.
"V'chol eilu peturim min hamaasros":
These vegetables are exempt from maaser during the other years of the shemittah cycle, the years in which the obligation of maaser applies, because they have the status of ownerless produce. And as is well known, the laws of Terumah and maaser do not apply to ownerless produce.
"V'nilkach mikol adam bashevi'is":
They may be purchased from anyone, even from an am ha'aretz who is not careful to properly observe the laws of shemittah. The reason is "she'ein kayotzei bahem nishmar" - anything similar to these items is not guarded, but is rather treated as ownerless. Therefore one may be lenient and buy it even from someone who will not handle the money he receives for it properly, even though that money takes on the sanctity of shevi'is.
A beraisa in the Gemara in Maseches Sukkah defines the scope of this leniency: even though one may buy from anyone, and even though the money given to him takes on the sanctity of shevi'is, this applies only to small quantities. With a small quantity - enough for three meals - we assume that even an am ha'aretz will use the money with the sanctity of shevi'is. Beyond that amount, however, one may not buy from him, since there is no certainty that he will use that money with the sanctity of shevi'is.
Two limitations emerge from this:
The leniency rests on the fact that these items are generally considered ownerless. Had they been the sort of things that people guard and keep in their possession, it would not be permitted to buy even a small quantity from an am ha'aretz, because of the possibility that he will not use the money with the sanctity of shevi'is.
Even with produce that grows in a manner that leaves it ownerless, the leniency applies only to an ordinary am ha'aretz. But someone whom we know to be openly suspected of doing business with shevi'is produce or of selling shevi'is produce - one may not buy anything from him.
Sefichin:
The mishnah continues and deals with sefichin - plants that grow on their own from seeds that fell before shevi'is. The Tannaim disagree about their status:
"Rabbi Yehudah omer: sefichei chardal mutarin" - mustard plants that grew on their own may be bought from anyone, "she'ein nechshedu aleihen ovrei aveirah" - those who transgress the laws of shevi'is are not suspected of bringing them from guarded fields of mustard; rather we assume they grew on their own and are ownerless.
"Rabbi Shimon omer: kol hasefichin mutarin" - anything that grows on its own may be bought from anyone, since we assume that all of it is ownerless, "chutz misefichei kruv" - except for cabbage that grew, "she'ein kayotzei bahem b'yarkos sadeh" - because such plants do not normally grow on their own in a field, and therefore we assume they were planted in a garden.
"Vachachamim omrim: kol hasefichin asurin" - all sefichin are forbidden, and they may not be bought from an am ha'aretz, out of concern that they were guarded inside a garden.
In summary: This mishnah lists wild vegetables that have the status of ownerless produce, and are therefore exempt from maaser and may be bought during shevi'is from anyone - but only in a small quantity and only from an ordinary am ha'aretz. At the end, the Tannaim disagree about the law of sefichin: Rabbi Yehudah permits mustard sefichin, Rabbi Shimon permits all sefichin except cabbage sefichin, and the Chachamim forbid them all.