1) WHAT PRODUCE IS SUBJECT TO THE OBLIGATION TO SEPARATE "TERUMOS U'MA'ASEROS"?
OPINIONS: The Gemara (end of 53b) cites a Mishnah in Terumos (2:4) that states that one may not separate Terumah from an item of one species on behalf of an item of a different species. The Gemara teaches that this is derived from the verse, "Kol Chelev Yitzhar v'Chol Chelev Tirosh v'Dagan" -- "The dedicated choice portion of oil and the choice portion of wine and grain" (Bamidbar 18:12), which teaches that the choice portion of each species must be given as Terumah for its own species and not for any other species.
RASHI (DH Ten Chelev) explains that "Chelev" refers to Terumah and Ma'aser which must be given to the Kohen. The Torah teaches that the Terumah and Ma'aser of olive oil, wine, and grain must be given separately.
The Gemara later asks, "How do we know that any two items of two other species may not be separated for each other?" Rashi (DH u'Shnei) writes that the Gemara's question is, how do we know that two species which are not grain -- such as beans and lentils -- cannot be used as Terumah for each other. The Gemara answers that the obligation to separate Terumah and Ma'aser for other species of produce is only mid'Rabanan. When the Rabanan instituted the requirement to separate Terumah and Ma'aser for these items, they applied the same criteria that apply to Terumah and Ma'aser d'Oraisa. Just as one may not separate from one species for a different species for Terumah d'Oraisa, one may not separate from one species for another for Terumah d'Rabanan.
The Gemara clearly implies that the obligation to separate Terumah and Ma'aser from all types of produce other than Dagan, Tirosh, and Yitzhar is only mid'Rabanan. Is this the Halachah?
(a) Rashi (DH Hani) explains that the obligation to separate Terumah and Ma'aser mid'Oraisa applies only to Dagan, Tirosh, and Yitzhar, as mentioned in the verse that the Gemara cites. All other species are obligated only mid'Rabanan. (See also Rashi to Kidushin 3a, DH Mipnei.)
TOSFOS (DH u'Shnei) also writes that all species that are not mentioned in the verse need to be tithed only mid'Rabanan. This is also the opinion of the RA'AVAD (Hilchos Ma'aser 1:9).
(b) The RAMBAM (Hilchos Terumos 2:1) writes that "any food that is fit for human consumption, and which is guarded, and which grows from the ground, is subject to the obligation of Terumah. It is a positive Mitzvah to separate the first part of it for the Kohen, as the verse says, 'The first of your grain, wine, and olive oil... you shall give to him (the Kohen)' (Devarim 18:4). Just as grain, wine, and olive oil are fit for human consumption and grow from the ground and have owners... so, too, any similar species is liable for Terumos and Ma'aseros." The KESEF MISHNEH writes that it appears that, according to the Rambam, all species of produce are liable mid'Oraisa. The Rambam understands that the Torah does not mean that only Dagan, Tirosh, and Yitzhar are obligated, but that all species similar to them are obligated.
However, the Rambam continues (Hilchos Terumos 2:6) and writes that "vegetables, even if they are fit for people, are liable for Ma'aser only mid'Rabanan, because the Torah states, 'The produce (Tevu'ah) of your seeds' (Devarim 14:22), and vegetables are not included in the category of 'Tevu'ah' and therefore are exempt mid'Oraisa."
(Tosfos here (end of DH u'Shnei) also writes that although all species other than Dagan, Tirosh, and Yitzhar are exempt mid'Oraisa, the Halachos for vegetables are more lenient, in certain ways, than the Halachos for legumes and fruit. Produce such as Kitniyos (e.g. beans, peas, rice) and fruit have an Asmachta in the verse to support the obligation to separate Terumos and Ma'aseros from them, while vegetables have no such Asmachta.)
The OR SAME'ACH (Hilchos Terumos 2:1, end of DH Ulam ha'Sugya) suggests that perhaps the Rambam maintains that for any plant from which one is obligated to give a portion to the poor or to the Kohen, one may not separate from one species for another, as derived from the obligation of Terumah of Dagan. However, with regard to vegetables, for which there is no obligation to give to the poor or to the Kohen, one might have thought that he may separate from one species for another. The Rambam perhaps understands that the Gemara's question here (how do we know that any two items of two other species cannot be separated for each other?) refers only to different species of vegetables. Accordingly, when the Gemara answers that these are obligated only mid'Rabanan, the Gemara refers only to vegetables. The Gemara, therefore, does not necessarily conflict with the Rambam's ruling that all species other than vegetables are obligated mid'Oraisa.
HALACHAH: The CHAZON ISH (Shevi'is 7:25, DH ul'Inyan) writes that it seems that the Halachah follows the majority of authorities who maintain that only Dagan, Tirosh, and Yitzhar are obligated mid'Oraisa. However, the MINCHAS CHINUCH (507:1) notes that the SHULCHAN ARUCH (YD 331:13) cites only the Rambam's opinion. Since the Shulchan Aruch, in his commentary on the Rambam (the Kesef Mishneh), explains that the Rambam maintains that all species are obligated mid'Oraisa, it seems that the Shulchan Aruch's opinion is that other species, and not only Dagan, Tirosh, and Yitzhar, are obligated mid'Oraisa. (D. BLOOM)
2) SEPARATING MA'ASER BEHEMAH "IN THE MIDDLE"
QUESTION: The Mishnah teaches that when there is a distance of 32 Mil or more between flocks of sheep, the flocks do not join together for the obligation of separating Ma'aser Behemah. (For example, if one has four sheep in one location and six in another, they are not considered ten sheep that must be tithed.) If, however, there is another flock of sheep between the two distant flocks, then they all join together and "are brought to the middle to be tithed." RASHI (DH Hayah) explains that this phrase is only figurative; it does not mean literally that the owner must bring together all the sheep when he separates Ma'aser Behemah.
Why does Rashi avoid the simple way of understanding the Mishnah? The Mishnah seems to be saying that the owner literally must bring all of the animals to the middle and separate Ma'aser Behemah from them there. Moreover, this would seem to be the logical requirement, since separating Ma'aser Behemah requires that one count all of the animals by having them pass through the gate of the pen one at a time.
ANSWER: As the BARTENURA writes explicitly, Rashi does not mean that it is not necessary for the flocks to be joined together in order to separate Ma'aser Behemah from them. Rather, he means that they do not have to be brought specifically to the middle. All of the animals may be brought to any of the flocks, even to either of the two flocks at the extremes. (Rashi's words, "Lav Meivi Mamash Ka'amar" should perhaps read, "Lav Meivi b'Emtza Mamash Ka'amar.")