The Mishnah (7:6) teaches that if one makes a neder prohibiting produce, then not only is the produce itself prohibited but also if they are exchanged or planted. In other words, if they are exchanged for an item, then that item becauses prohibited as well. The Bartenura explains that once an object because assur be'hanah, then the Chachamim applied it to its value as well. The Mishnah also explains that if the produce is planted, then that which grows from those seeds (gidulin) is also prohibited.1
The Tosfot Yom Tov cites the Raavad who asks why the gidulin should be prohibited. There are two contributing facts that allowed the seed to grow, the seed itself which is indeed prohibited and the ground which is permitted. Consequently, since both prohibited and permitted factors combine in production it should be permitted – ze ve'ze gorem.2
The Kesef Mishnah answers that this case is different. Something prohibited by a neder is defined as a davar she'yesh lo matirin – a prohibition that has a means of being permitted. This is because, as we have learnt, one can go to a Chacham to matir (undo) a neder. Normal laws of bitul (nullification) do not apply to a davar she'yesh lo matirin since there is a way of permitting the prohibition without needing to rely on bitul. Consequently, in our case as well, since it is a davar she'yesh lo matrin, ze ve'ze gorem does not help to permit the gidulin.
The Tosfot Yom Tov however notes that tevel (untithed produce) is also defined as a davar she'yesh lo matrin. Nevertheless, we find that if tevel is planted, the gidulin are permitted. The case of planted tevel appears to contradict the Kesef Mishnah's explanation.
The Tosfot Yom Tov answers that a majority of tevel (once tithed) is chulin (regular produce). Consequently, given that the stringencies applied to davar she'yesh lo matirin are rabbinic, they did not rule stringently in the case of tevel.
The Bartenura however explains the gidulin are prohibited much like gidulei hekdesh. We have discussed previous that when one makes a neder, they make the object prohibited in a similar manner that hekdesh (property consecrated to the Beit HaMikdash) become prohibited. It appears that the Bartenura reasons that just like gidulei hekdesh are prohibited, the world of nedarim also adopts that law.3
Common to the answers thus far is that nedarim is an exception to the principle of ze ve'ze gorem. The Tosfot however help us understand ze ve'ze gorem in general. They explain that the law only applies when the contributors are of the same type and have the same effect. For example, when both permitted and prohibited sourdough combined to cause dough to rise. In this case the seed and ground act differently. Consequently, this case cannot be defined as ze ve'ze gorem .4
1 The law is the same whether or not the seeds disintegrate (e.g. wheat) or that which is planted remains (e.g. onions). The distinction between types of seeds is only important when one makes a neder not to eat specific produce as explained towards the end of the Mishnah.
2 We have looked at the issue of ze ve'ze gorem previously in the context of orlah. See Volume 13, Issue 44.
3 This is also one of the answer presented by the Tifferet Yisrael.
4 See Volume 13, Issue 44 where this answer is probed in more detail.
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