Kilayim, Chapter Six, Mishnah Five
Our mishnah defines what is a non-fruit bearing tree, termed in Hebrew serak, with regard to the halakhot which we learned above in mishnayot three and four.
What is a serak (non-fruit bearing) tree?
1) Any tree which does not yield fruit.
2) Rabbi Meir says: all trees are serak, except the olive and the fig tree.
3) Rabbi Yose says: all trees that are not planted in whole fields, are serak trees.
Section one: The first opinion simply holds that all trees that do not bear fruit are considered to be serak trees, and all of the rules in mishnah three above apply to them.
Section two: Rabbi Meir holds that the only non-serak trees, meaning the only trees whose fruit are significant, are the olive and fig trees. All other trees are considered serak, non-fruit bearing. We should probably remember that the wide variety of trees that we now have certainly did not exist in the ancient near east. The one significant tree that Rabbi Meir does seem to be excluding is the date palm. Perhaps date palms were not used in training grape vines and hence Rabbi Meir just doesnt consider them in this context. Alternatively, Rabbi Meir holds that grapes were more valuable than dates, and hence compared to the vine, the date palm is non-fruit bearing.
Section three: Rabbi Yose says that a tree is considered to be a fruit-bearing tree if people will plant a whole field of that type of tree. For instance, if people will plant a whole field of etrog trees, then an etrog tree would be considered fruit bearing in the context of the rule above in mishnayot three and four. If not, then it is considered to be serak tree.