1) AVOIDING "KIL'AYIM" ON TWO SIDES OF A FENCE
QUESTION: The Gemara cites the Mishnah later (26a) which states that when two fields are separated by a fence, the owners of both fields may plant trees adjacent to the fence. Similarly, when one of the fields is a wheat field, the owner of the other field may plant trees adjacent to the fence and he does not transgress the Isur of Kil'ayim.
How does a fence prevent a person from transgressing the Isur of Kil'ayim? The roots of the trees will meet the roots of the wheat below the fence, and thus it still should be prohibited to plant both plants adjacent to each other, even if they are separated by a fence.
(a) TOSFOS later (19a, DH ha'Mavrich) proves from this Gemara that the Isur of Kil'ayim applies only when the plants mingle above the ground. The Isur of Kil'ayim does not apply to plants that are separated above the ground, even if their roots mingle below the ground. This also seems to be the approach of RASHI here (end of DH Geder).
(b) The RI MI'GASH writes that the Isur of Kil'ayim requires two things: that the roots of the two plants not absorb their nourishment together, and that the offshoots and branches of the two plants not mingle. The Mishnah refers to a case in which there is a large rock ("Tzunma") beneath the fence. The rock prevents the roots of the plants from mingling, and the fence between the two plants prevents their branches from mingling. (This view is also cited by the ME'IRI at the beginning of Bava Basra.)
The KOVETZ SHI'URIM points out that according to this explanation, when the Gemara in Eruvin (11b) says that the Halachic concepts of "Tzuras ha'Pesach" and "Gud Achis Mechitzasa" create a separation between plants, which prevents one from transgressing the Isur of Kil'ayim, it must refer exclusively to a case in which there is a large rock between the roots of the plants, because a separation above the ground is not sufficient to avoid the Isur of Kil'ayim (according to the Ri mi'Gash).
(RAV SHLOMO ZALMAN AUERBACH zt'l in MINCHAS SHLOMO (end of #36) questions this explanation based on the Gemara later (19b) which says that when one replants a shoot of a grapevine, wheat seeds may not be planted directly over it but may be planted on the sides, even within a distance of three Tefachim, even though the roots of the plants will mix. The Gemara there certainly cannot refer to a case in which the roots are separated by a rock, because the Gemara there attempts to derive from that case that a plant's roots do not spread out sideways.) (I. Alsheich)
2) THE BEES AND THE MUSTARD
QUESTION: The Gemara explains that the Rabanan maintain that a person who raises bees to produce honey is entitled to prevent his neighbor from planting a mustard plant near the boundary of his own property. A mustard plant is considered a damaging object because when the bees eat the mustard plant its sharp taste causes them to eat their own honey, and thus the owner will lose his honey. The Gemara asks, according to the Rabanan, why is the owner of the bees not similarly required to distance his bees from the boundary since, after all, they will harm the neighbor's mustard plant when they eat it? The Gemara answers that bees do not cause damage to a mustard plant. It is very difficult for them to find the kernels, so it is not likely that they will damage the mustard plant by eating the kernels. Although they will eat the leaves, that is not considered damage because the leaves will grow back.
How does the Gemara answer its question? Even if the leaves grow back, the bees still will cause damage to the mustard plant, because when the leaves grow back the bees will eat them again. The bees should be considered a damaging agent because they can be expected to eat the leaves of the mustard plant every time the leaves grow back, and thus the mustard plant will never be able to grow properly. (RA'AVAD, cited by RAMBAN and RASHBA)
(a) The RA'AVAD answers that once the bees have eaten the mustard leaves once and have tasted their sharpness, they will not eat them again. Since the leaves will grow back and the bees will not eat them again, the owner of the bees is not required to prevent his bees from eating the mustard leaves. The owner of the mustard plant, on the other hand, must remove his plant from the boundary, because even if the bees eat the leaves only once they will have a desire to eat their own honey, and their owner will lose some honey as a result.
The RASHBA challenges this answer. The Gemara (end of 18a, in its explanation of the view of Rebbi Yosi) implies that the bees constantly come to eat the mustard plant ("Ba'os v'Ochlos"); they do not eat from it just once and not more.
Moreover, the Gemara implies that even after the mustard plant has been planted, the Rabanan require the owner to uproot his plant and plant it farther away from the boundary. According to the Ra'avad's explanation, why does the owner have to relocate his plant? The Ra'avad maintains that once the bees have eaten from it the first time, they will not eat from it again. Since they will not eat from it again, the plant no longer poses a threat to the owner of the bees. According to the Ra'avad, the owner of the plant should be allowed to leave his plant adjacent to the boundary.
The Rashba asks further that according to the Ra'avad, the Rabanan's rejoinder to Rebbi Yosi is difficult to understand. The Rabanan explain that the bees are not considered a Mazik to the mustard plant because the leaves that they eat will grow back, whereas the mustard plant is considered a Mazik to the honey. If the bees eat the leaves of the mustard plant only once, however, then the mustard plant itself can cause damage only once, for its harmful influence occurs only when the bees eat its leaves. Before the bees eat the mustard leaves, both the bees and the plant should be considered potential Mazikin to each other. After the bees have eaten the leaves once, neither of them should be considered a potential Mazik, because the bees will no longer eat the leaves. Why, then, is there a distinction between the bees and the leaves?
(b) The RASHBA answers the Ra'avad's question in a different way. The leaves of a mustard plant are picked as soon as they grow. Thus, the leaves will be picked before the bees can eat them. Although the bees might eat a little bit before the owner has a chance to pick the leaves, the small amount that they consume is not considered a loss at all.
(c) The Rashba gives another explanation. The primary function of the leaves is to protect the kernels of mustard. Since the leaves grow back whenever the bees eat them, the main part of the mustard plant -- the kernels -- will not sustain any damage, because it will always be protected by leaves. Hence, even if the bees continuously eat the leaves as they grow back, they will not cause any damage because the kernels will always be protected, and the leaves themselves have no intrinsic value. (I. Alsheich)