Orlah, Chapter 1, Mishnah 7. This mishnah discusses the various parts of a fruit tree, and the question of which of them are subject to the laws of orlah and which are not.
Four parts that do not have the status of fruit:
"He'alim" - the leaves of the tree.
"Veha'lulavim" - the very soft, young shoots.
"U'mei gefanim" - the sap that flows from the vine when it is cut during the month of Nisan.
"Vehasemadar" - the tiny fruit that appears once the blossom has fallen off, a very small fruit that holds within it the potential to grow.
All of these items are "mutarin" - permitted, in three areas:
In orlah - they are not subject to the laws of orlah.
And in reva'i - even in the fourth year they are not subject to the laws of neta reva'i, which requires bringing the produce up to Jerusalem.
And for a nazir - a nazir, who is forbidden to eat grapes and anything that comes from grapes, may partake of them, since they are not considered grapes.
"Ve'assur ba'asheirah":
However, if we are dealing with a tree or a vine that was worshipped as idolatry, which is called an asheirah, all of these items are forbidden as part of the tree. The reason: regarding an asheirah and idolatry the Torah says "lo yidbak beyadcha me'umah min hacherem" - let nothing of what is banned remain in your hand, not even the smallest thing. Therefore even the slightest benefit, even the most incidental benefit, is a problem, and this is what sets an asheirah apart from all other prohibitions.
The view of Rabbi Yosei:
Rabbi Yosei says: "hasemadar assur" - the semadar is forbidden under the laws of orlah. The semadar, as explained, is the tiny fruit that appears immediately after the blossoms fall, and in Rabbi Yosei's view it has the status of fruit - it is the fruit itself, only that it has not yet developed.
The dispute between Rabbi Eliezer and Rabbi Yehoshua regarding a curdling agent:
First we need to understand the concept of a curdling agent: in the process of making cheese, various substances were used to curdle the milk. Apparently, when rennet was not available, or in order to assist the action of the rennet, other substances were used, some of them by-products of fruit.
Rabbi Eliezer says: "hama'amid bisraf orlah - assur" - one who curdles cheese with the sap of an orlah tree, it is forbidden.
Rabbi Yehoshua said: "shamati beferush" - I heard explicitly, and he drew a distinction between two kinds of sap:
"Shehama'amid bisraf alim uvisraf ikarim - mutar" - one who curdles cheese with sap that flows from the leaves or with sap that flows from the roots, it is permitted, since such sap is not considered part of the fruit.
"Bisraf hapagim - assur" - pagim are unripe fruits, fruits that have not yet fully ripened, and the sap that flows from them is forbidden, "lefi shehu peri" - this sap has the status of fruit, since it comes out of the fruit itself.
In summary: in this mishnah we learned that the leaves, the young shoots, the vine sap and the semadar are not considered fruit, and are therefore permitted with regard to orlah, reva'i and a nazir, while with regard to an asheirah they are forbidden because of "lo yidbak beyadcha me'umah min hacherem." Rabbi Yosei disagrees and forbids the semadar, since it has the status of fruit. Regarding a curdling agent, Rabbi Eliezer forbids curdling with the sap of an orlah tree, and Rabbi Yehoshua distinguishes: sap of the leaves and roots is permitted, since it is not part of the fruit, while the sap of unripe fruits is forbidden, since it comes out of the fruit itself.