Uktzin, Chapter 1, Mishnah 6. The subject of this chapter is those portions of a food that nobody actually eats: the stalks, stems, twigs and pods by which produce is picked up and carried. A part like this can be classified as a yad, a handle serving the food, and if it is, it becomes tamei along with the food and passes tumah on to it. Our mishnah adds a further set of examples in which some growth still joined to produce helps a person grasp it, and the question in each case is whether that growth retains handle status.
"Kol yados ha'ochlin shebasasam bagoren, tehorin": all handles of foods that were threshed on the threshing floor are tahor. Picture stalks of wheat with the kernels still on them that have gone through the threshing process. The purpose of threshing is precisely to separate the grain from the stalks, and although some kernels may remain attached, the stalks themselves have been split, cracked and frayed in the process. They can no longer serve a person for gripping and lifting the grain, and once they have lost that usefulness they no longer carry the halacha of a yad for the food.
"Rabbi Yosei metamei": Rabbi Yosei rules that they remain tamei, consistent with the view he expressed earlier in the chapter. To his mind, even these beaten stalks are still being put to work. When kernels are left clinging to a stalk and one wants to dislodge them, he raises the straw with a pitchfork and flings it about; the fork grips the straw, so the straw is what enables the grain still hanging on it to be moved, which is exactly the function of a yad.
"Psigah shel eshkol sheriknah, tehorah": imagine a small cluster of grapes branching off a large cluster growing on the vine. If that little offshoot has been emptied, meaning that its grapes were all picked off, the bare twig that remains is tahor. It serves no purpose any longer, so it is not a handle. "Shiyeir bah garger echad, temeiah": but if he left even a single grape on that little branch, it is tamei, because he will use the branch to take hold of that one grape and lift it.
"Shavit shel temarah sherikno, tahor": the identical rule applies to the twig of a date palm from which the dates were stripped. Emptied of its fruit, it is no longer a handle and is tahor. "Shiyeir bo temarah achas, tamei": if he left one date on it, the twig is tamei, since it remains a handle for that single date.
"V'chein bekitniyos": so too with legumes that grow on a stalk inside pods that house the beans or peas. "Shavit sherikno, tahor": if the pod was emptied out entirely, it is tahor and is not treated as a handle. "Shiyeir bo garger echad, tamei": if he left one bean inside the pod, it is tamei.
The final ruling in the mishnah, "metaher beshel pol umetamei beshel kitniyos, mipnei shehu rotzeh bemishmushen", is stated in the name of Rabbi Elazar ben Azaryah, who distinguishes between species. Regarding the pol, one specific type of bean, he declares the pod tahor: beans of this kind are big, so nobody relies on the pod in order to pick them up; the bean is grasped directly. Among the smaller legumes, though, he concedes that the pod does function as a handle and is tamei, for there a person genuinely wants the pod when handling the produce, and it is that want which gives it the standing of a yad.
The thread running through all of these cases is the same: a stem, twig or pod is a handle only as long as it actually helps a person take hold of food. Empty it of its fruit, or leave the fruit large enough to be grasped on its own, and the attachment reverts to being an ordinary piece of wood or husk that neither receives nor conveys tumah.