Uktzin, Chapter 1, Mishnah 3. Maseches Uktzin deals with food that has become tamei and with its ability to transmit that tumah onward, and its central concern is defining which parts of a food item count as the food itself. The masechta opened by establishing three categories: a yad, a part that serves as a handle for holding the food while eating, which becomes tamei together with the food and conveys tumah to it, but does not combine with it toward the shiur of a kabeitzah required to transmit tumah further; a shomer, a part that protects the food and preserves it, which is treated as the food itself in every respect, including combining for the shiur; and a part that is neither, which is excluded from these laws entirely. Our Mishnah continues the list of items that qualify as a shomer.
The Mishnah teaches: "Shoroshei hamisnah vehapigam." These are the roots of the misnah, which some read as hamintsa, a species of herb, and of the pigam, a plant that bears yellow petals.
"Veyarkos sadeh veyarkos ginah she'akaran lishoslan." Here we are speaking of the roots of various vegetables that grow in the fields or in gardens, in a case where a person uprooted them with the intention of replanting them elsewhere. Since replanting is his purpose, it matters a great deal to him that the roots remain whole and undamaged, and that importance is what gives them the standing of a shomer.
"Vehashidrah shel shiboles vehalevush shelah." Also included is the stalk of the wheat and the husk that surrounds the grain. All of these are considered a shomer for the food, so that if such a part becomes tamei it conveys tumah to the produce, and it is reckoned together with the produce as well.
An additional inclusion is recorded in the name of Reb Elazar, who speaks of the "siyag shel rutzifas." As such plants are pulled out of the ground, clumps of soil come up clinging to them. That soil has value in the owner's eyes and it shields the vegetable, so the very same ruling applies to it as well.
The Mishnah concludes: "Harei eilu mitamin umitamin umitztarfin." All of these become tamei together with the food, convey tumah to the food, and combine with it to complete the shiur of a kabeitzah, exactly as a shomer does, with all the halachos set out in the Mishnah applying to them in full.