Before us is Mishnah 6 in chapter 7 of Maseches Shevi'is. The Mishnah lists several species and discusses whether the laws of Shevi'is apply to them:
"HaVered" - the wild rose.
"VehaKofer" - cypress.
"VehaKetaf" - we will discuss its identity further on.
"VehaLotem" - lotus.
"Yesh lahen shevi'is, ul'dmeihen shevi'is" - the laws of Shevi'is apply to them, and to the money received for them as well, because they are considered food, that is, something fit for eating.
The opinion of Rabbi Shimon:
"Rabbi Shimon omer: ein l'ketaf shevi'is, mipnei she'eino peri" - in his view the ketaf is a kind of sap that flows from the tree, from the balsam tree, and the laws of Shevi'is do not apply to it. This sap is not a fruit but part of the tree itself: it is not considered produce of the tree but the tree itself, and therefore the laws of Shevi'is do not apply to it.
Here, then, is the basis of the dispute: Rabbi Shimon holds that the laws of Shevi'is do not apply to the ketaf, as opposed to the first Tanna, who holds that it is considered a kind of fruit of the tree, and since it is the fruit of the tree the laws of Shevi'is apply to it.
In summary: in this Mishnah we learned that the rose, the kofer, the ketaf and the lotem are subject to the laws of Shevi'is, as is the money received for them, because they are fit for eating. Rabbi Shimon disagrees with regard to the ketaf alone, since in his view it is not a fruit but sap that flows from the tree, and its status is that of the tree itself rather than its produce.