Uktzin, Chapter 1, Mishnah 2. Maseches Uktzin is devoted to foods that have become tamei, to their power to pass tumah along to other foods and utensils, and to the question of which portions of a fruit or vegetable are reckoned part of the food. The first Mishnah of the masechta established three classifications. A yad is a portion that serves as a grip, by which one holds the item while eating it; it contracts tumah together with the food and passes tumah to it, yet it is not counted together with the food when we measure the kabeitzah required for transmitting tumah onward. A shomer is a portion that shields the food and keeps it from spoiling, and it is treated exactly like the food in all respects, including being counted in that measure. And a portion that is neither of these lies wholly beyond these halachos. Our Mishnah proceeds to identify which parts of various vegetables have the status of a shomer.
The Mishnah lists: "Shoroshei hashum vehabetzalim vehakaflotos, bizman shehein lachim." The roots of garlic, of onions and of similar vegetables are a shomer, but only while they are still lachim, moist and fresh.
"Vehapitmah shelahen, bein leicha bein yeveisha." The pitmah is the small protrusion at the top, and regarding it there is no such condition: whether it is still moist or has already dried out, it is a shomer.
"Veha'ometz shehu mechuvan keneged ha'ochel." As an onion grows old, a stalk emerges from it which holds the seeds within it, and this is likewise a shomer when it is situated directly across from the edible portion. Others understand the ometz differently, as referring to the grassy shoots that sprout upward from the onion.
"Shoroshei hachazarin vehatznon vehanafas." Also included are the roots of the romaine lettuce, the roots of the radish, and the nafas, which is a species of radish. "Divrei Reb Meir." All of this is the opinion of Reb Meir, who holds that each of these items functions as a shomer: as long as it remains whole and attached, it protects the produce, and once it is cut away the vegetable spoils more quickly. That is precisely what makes it a shomer, so that it becomes tamei with the food, conveys tumah to the food, and joins with it toward the kabeitzah.
Reb Yehuda takes a far more limited view. "Reb Yehuda omer", and he rules: "shoresh tzonon gadol mitztaref." In his opinion it is only a large radish's root that has the status of a shomer and is therefore counted together with the vegetable when we measure whether there is a kabeitzah. "Vehasiv shelo einam mitztaref." The thin hairlike fibers growing off that root, however, are excluded; they have no status of shomer and are not added to the measurement.
This Mishnah thus brings us down from the theoretical definitions to actual produce held in the hand: the roots, tips and shoots of garlic, onions, lettuce and radishes. Reb Meir finds in each of them a real protection for the vegetable, while Reb Yehuda confines the category of shomer to a single case, the root of a radish that is large, and even there he leaves out its fibers.