Gemara

73b (Rava): If one gathers salt from Misrefos (place where they evaporate seawater to make salt), he is liable for Me'amer (making piles).

(Abaye): [He is exempt.] Me'amer applies only to Gidulei Karka (things that grow from the ground).

96b (Rav Yehudah): The [man executed for] Mekoshesh (gathering wood, Bamidbar 15:32) carried four Amos in Reshus ha'Rabim.

(Beraisa): He detached wood [from trees].

(Rav Acha b'Rebbi Yakov): He was Me'amer.

142a (Rabah bar bar Chanah): One may carry a basket full of produce with a rock inside.

Question: He should spill out the produce and rock, and return the produce to the basket!

Answer: The case is, the produce would get dirty.

143b (Beraisa): If produce scattered in a Chatzer, he gathers it bit by bit and eats. He may not gather into a basket or box, for this is like one does on a weekday.

Beitzah 33b (Beraisa): One may pile up (straw) from the Chatzer and burn it, as long as he does not make piles;

R. Shimon permits.

The first Tana forbids, for it looks like he makes piles for other days. R. Shimon permits, for his pot proves that he does so for now [for cooking].

Rishonim

Rambam (Hilchos Shabbos 8:6): One who gathers pressed dates and makes a ring of them, or punctured figs and puts them on a string, and gathers and makes one mass of them, he is liable for a Toldah of Me'amer. The same applies to all similar cases.

Rambam (21:11): If one sticks together Peros until they become one mass, he is liable for Me'amer. Therefore, if produce scattered in a Chatzer, he gathers it bit by bit and eats. He may not gather into a basket or box, like one does on a weekday, lest he press them with his hand into the box and come to Imur. Similarly, one may not gather salt or similar things, for it looks like Me'amer.

Magid Mishneh: The Rambam explains that one may not gather into a box, lest he come to Imur. This is a distant concern. This requires investigation. The Rashba brings in the name of his Rebbi (R. Yonah) that this is if they were scattered in the Chatzer, one here and one here. If they were gathered in one place, he may gather them into a basket. The Ramban says that they fell in the Chatzer amidst pebbles and earth. One may not gather them because it looks like Borer. The Mishnah discussed in the Chatzer, for a house is often swept, and pebbles and earth are not common there. A Tosefta (17:6) supports this. The Rashba wrote similarly in his Sefer (Avodas ha'Kodesh 1:5).

Rambam (25:20): If there is Tamei Terumah and Chulin in a Kli, he may not carry them if he could spill it out and the food would not get ruined. He spills it out, and takes the Chulin.

Ran (60a DH Nispazru): Why is it considered Uvda d'Chol (a weekday activity) to gather Peros? Above, the Gemara suggested that he should spill out the produce and rock, and return the produce to the basket! R. Yonah answered that they were scattered in the Chatzer, one here and one here. If they were gathered in one place, he may gather them into a basket.

Rosh (7:2): The Halachah follows Abaye (who exempts for gathering salt from Misrefos).

Rosh (22:1): The Beraisa says 'he gathers it bit by bit and eats.' It seems that the text should not say 'and eats', for the Seifa says 'he may not gather them into a basket.' This implies that only this is forbidden, due to Uvda d'Chol, but one may gather into his lap or garment.

Rashi (96b DH Me'amer): He gathered scattered, detached branches. This is like we learned, that one who gathers salt is liable for Me'amer.

Minchas Chinuch (32, Musach ha'Shabbos 4 (Me'amer)): Rashi teaches that Me'amer is not limited to food. One could have explained Beitzah 33b to teach, like the Rambam, that it is limited to food. Rather, Rashi explains like Tosfos (Beitzah 31a DH Min), that it applies only where the matter grew. The Rambam holds that the Mekoshesh carried in Reshus ha'Rabim, or detached.

Note: The Gemara said that each opinion teaches a Melachah for which one is killed. According to the Minchas Chinuch, it could have said that they argue about whether Me'amer is limited to food.

Poskim

Shulchan Aruch (OC 335:5): If produce scattered in a Chatzer, one here and one there, he gathers it bit by bit and eats. He may not gather into a basket or box.

Beis Yosef (DH uvi'Semag): Semag says that one must be careful not to gather Peros together, for this is like Me'amer. However, Beitzah 33b proves that Me'amer is only if one took them from where they grow and gathers them together, like a standing crop that was cut. One piles up the sheaves.

Mishnah Berurah (17): It is Uvda d'Chol to gather and put them in a basket, but not if they fell in one place. However, if they fell into pebbles or dusty earth, in every case it looks like Uvda d'Chol to gather and put them in a basket. It is forbidden even if he intends to eat them immediately. If he does not intend to eat them immediately, it is forbidden due to Borer!

Gra (DH v'Ochel): The Gemara, Rif, Rambam, Ran and Magid Mishneh say that he gathers and eats, unlike Tosfos and the Rosh.

Bi'ur Halachah (DH Echad): Do not say that the Gra discusses only when they became mixed with pebbles. He brought the text of the Beraisa 'and he eats', unlike Tosfos and the Rosh. Therefore, according to R. Yonah, who explains that in the Beraisa they scattered in different places, also then we may not be lenient. Perhaps the Beis Yosef retracted, for in the Shulchan Aruch he wrote 'and eats.' Surely one may not be lenient to gather them into his lap or garment when they became mixed with pebbles or earth. The Pri Megadim says that this is Borer. Presumably, one may gather them to eat them immediately. However, it seems that the Gra is stringent in every case.

Kaf ha'Chayim (28): Ben Ish Chai brings from Nezirus Shimshon that one who relies on Tosfos and the Rosh does not lose, for it is a mid'Rabanan matter.

Kaf ha'Chayim (31): On Simchas Torah, they scatter fruits and children gathered them into baskets. Since they do so for Simchah, we are not concerned, like we say about Purim. (Rema 696:8 favors the opinion that permits wearing clothing of the opposite gender, or Kil'ayim mid'Rabanan, since it is for the sake of Simchah.)

Shulchan Aruch (ibid.): If they fell in one place, he may gather even into a basket.

Kaf ha'Chayim (32): He may put in the basket because it is not an exertion.

Shulchan Aruch (ibid.): However, if they fell into pebbles or dusty earth in the Chatzer, he gathers one by one and eats. He may not gather into a basket or box.

Mishnah Berurah (18): According to the Gra, we forbid gathering into his lap or garment. The Mishnah mentioned into a basket or box, for this is a typical case.

Kaf ha'Chayim (34): The Levush says that putting in a basket is like Borer. Rabbeinu Zalman says that it is like Uvda d'Chol.

Shulchan Aruch (340:9) One may not gather salt from Misrefos, for it looks like Me'amer.

Beis Yosef (DH Asur): The Rosh and Rambam rule like Abaye, that one is exempt, for Me'amer applies only to Gidulei Karka. The Tur holds that Abaye agrees that it is forbidden. This is proper. We should not say that there is a vast argument between Abaye and Rava.

Magen Avraham (15): Olas Shabbos asked why he rules like Abaye against Rava. The Rosh's text says that Rabah argues with Abaye. (Note: the Bach and Korban Nesan'el change the text of the Rosh to say 'Rabah.') This is primary, for Rava himself (75a) says that Dishah applies only to Gidulei Karka. He rules like Abaye, for there is no source to distinguish Dishah from Me'amer, since we learn all the Melachos from the Mishkan. The Magid Mishneh says that also the Rambam's text said Rabah. The Halachah follows Abaye, for he is Basra.

Gra (DH b'Makom): In Beitzah, Tana'im argue about whether it looks like he makes piles for other days. No one forbids due to Imur, for it [applies only where the Peros grew,] like reaping.

Mishnah Berurah (35): Me'amer is gathering the sheaves. Gathering salt is similar, but only in the Misrefos, which is where it grows. If it was scattered elsewhere, one may gather it into one place.

Mishnah Berurah (36): Therefore it is forbidden mid'Rabanan. Imur itself is only with Gidulei Karka.

Kaf ha'Chayim (72): The same applies where there are salt reserves on the ground.

Shulchan Aruch (ibid.): Likewise, one may not gather anything from where it grows.

Mishnah Berurah (37): One is liable for fruits, vegetables, wood, herbs and all that grows, where they grow. The Mechaber said 'Asur' to teach that where it does not grow e.g. if Peros scattered in the house, it is permitted. People who guard gardens and orchards must be very careful not to gather even fruits that fell off the tree before Shabbos, even a small amount. One is liable for this, for the Shi'ur of Imur is very small. The Rambam says that if he gathers food to eat, for an animal to eat, or for fuel, the Shi'ur is the size of a dry fig, to fill the mouth of a kid, or enough to cook an egg, respectively.

Kaf ha'Chayim (74): Most Poskim say that Me'amer is only where the Peros grow. Keneses ha'Gedolah says that the Rambam and Mordechai disagree. The Acharonim and some Rishonim say that even the Rambam agrees. In any case, one should be careful when possible.

Shulchan Aruch (10): One who gathers pressed dates and makes a ring of them, or punctured figs and puts them on a string, and gathers and makes one mass of them, he is liable for a Toldah of Me'amer. The same applies to all similar cases.

Magen Avraham (16): This is only if he gathered them from where they fell from the tree, but not if he gathered them in the house. Making a pile is not the final Melachah, like it says in Beitzah.

Magen Avraham (17): Why isn't one who makes a ring of figs liable for building, like we say about one who makes cheese? We must say that building is only when he intends to beautify it and straighten it.

Mishnah Berurah (38): One is liable because it is normal to connect these fruits in this way. Nishmas Adam and Ma'aseh Roke'ach say that in such a case, one is liable even in the house.

Kaf ha'Chayim (80): The same applies to gathering pearls. Also they grow from the ground!