- כל מדות חכמים
On Daf 12b the Gemara brought a Mishnah that taught: התבואה והזיתים משיביאו שליש – Produce and olives require tithing from when they grow to one third of their maturity. Rashi explains, that produce refers to grains and grapes, based on the passuk, כתבאת גרן וכתבואת יקב – like produce of the threshing floor and produce of the wine vat. On this Daf, Rebbe Yirmiyah said to Rebbe Zeira: But could the Rabbanon determine the difference between a plant that just reached a third of its growth and one that reached just less than a third? Rashi explains that he was asking if they were such experts in a plant’s growth? Rebbe Zeira said to Rebbe Yirmiyah that he should not cast aspersions on the measurements of the Rabbanon, for כל מדות חכמים כן הוא – all the measurements of the Rabbanon are like that; they are given in exact terms. For example, one cannot immerse in a mikvah measuring forty se’ah minus a kortov, and an amount of food the size of an egg less the volume of a sesame seed אינו מטמא טומאת אוכלין – does not transmit food tumah.
- The source for determining the year of produce based on reaching one third of its maturity
The Gemara seeks the source for the rule that produce is assigned to a given year based on when it reaches one third of its maturity. It was taught in a Baraisa: Rebbe Yonasan ben Yosef says: The passuk states: I will ordain my berachah for you in the sixth year, "ועשת את התבואה לשלש השנים" – and it will make a crop sufficient for the three years. אל תקרי "לשלש" אלא "לשליש" – Do not read it as it is written: l’shalosh (for the three years), rather, read it as though it were written l’shlish (for a third of the crop), so that the sense of the passuk is that produce is considered finished when it is one-third ripe, and it is this moment that determines to which year’s crop it should be assigned. When the Gemara asks how this passuk can be used for a derashah when it is needed for the literal meaning, it answers that another passuk is written for that purpose: "וזרעתם את השנה השמינת ואכלתם מן התבואה ישן עד השנה התשיעת" – And you shall sow in the eighth year, and you shall eat from the old crop until the ninth year. Since this passuk already indicates that the crops of the sixth year will last for three years, until the beginning of the ninth year, there was no need for the Torah to further state that the sixth year will make a crop for three years.
- Kitniyos are determined based on when they take root
The Gemara brings a Mishnah in Sheviis that teaches: Rice, millet, poppy and sesame, which are all types of kitniyos, שהשרישו לפני ראש השנה – that took root before Rosh Hashanah, are tithed according to the past year and are permitted in the seventh year. But if they took root after Rosh Hashanah, they are forbidden during the seventh year and are tithed according to the coming year. Rabbah said: אמור רבנן אילן בתר חנטה – The Rabbanan said that the assignment of the fruit of a tree follows the emerging of its fruits, תבואה וזיתים בתר שליש – and produce and olives follow when they grow to a third of their full size, ירק בתר לקיטה – and a vegetable follows when it is picked. So what do the Rabbanon equate these items of kitniyos with? The rule the Mishnah gives them of taking root does not match any of these categories. Rabbah then said: מתוך שעשויין פרכין פרכין אזלי רבנן בתר השרשה – Since they are crumbled one at a time , meaning they are harvested a few at a time, resulting in some being picked before Rosh Hashanah and some after, the Rabbanon chose to assign them based on taking root.