At the end of 79a, Rashi and Tosfos (quoting R"Ch) have different opinions over whythe fish pond/forest with lumber wood would have to be sold and exchanged for karka that can give peiros. Could you lay out their 2 opinions?
Aaron Gavant, Baltimore, USA
Rashi holds that the fish-pond and the forest are considered Keren, because the husband will eventually finish off the fish, leaving over no Keren, and wherever the Peiros terminate, it is considered Keren.
Tosfos disagrees a) with Rashi's basic explanation of the Gemara's subject-matter, and b) with both the wording of the text and his definition of Peiros and Keren.
In Tosfos opinion, if the woman were to inherit either of the above, they would both be considered Peiros, on account of the land that remains after the fish and the wood are finished (even though it is only very little, as it is no worse than worn-out clothes, that are considered Keren, too).
And what the Gemara is talking about is (not what to do in the event that the woman inherited either of these, but) which of the two to purchase in the event that she inherited money. And it is in this connection (exclusively) that the one opinion considers whatever will re-grow to be Keren, and whatever will not, Peiros (in contrast to Rashi's text).
Consequently, when, according to this opinion, Chazal say that one should purchase Karka, one may only purchase something that will re-grow when it is finished (so that the original Keren remains intact) and not something that will not (even though it too, might well be considered Peiros in its own right, as we explained).
Be'Virchas Kol Tuv