Mishnah Uktzin 2:3: Regarding a pomegranate or melon, part of which has rotted, it [i.e. the rotten part] does not join together [with the rest of the food, to count towards the measure of impurity]. If it is whole on either side, and rotten in the middle, it does not join together [and even the sides are not joined to each other]. The stem of a pomegranate does join together [to the rest of the fruit], and its flower does not join together. Rabbi Elazar says: even the crown [literally: the comb, around the stem] is pure [and does not join together].

הָרִמּוֹן וְהָאֲבַטִּיחַ שֶׁנִּמּוֹק מִקְצָתוֹ, אֵינוֹ מִצְטָרֵף. שָׁלֵם מִכָּאן וּמִכָּאן וְנִמּוֹק מִן הָאֶמְצַע, אֵינוֹ מִצְטָרֵף. הַפִּטְמָא שֶׁל רִמּוֹן, מִצְטָרֶפֶת. וְהַנֵּץ שֶׁלּוֹ אֵינוֹ מִצְטָרֵף. רַבִּי אֱלִיעֶזֶר אוֹמֵר, אַף הַמַּסְרֵק טָהוֹר:

Mishnah Uktzin 2:4: All peels can be rendered impure, and render impurity, and join together [with the food to count towards the measure for impurity]. Rabbi Yehuda says: the onion has three peels; the inner one joins together, whether it is whole or pierced; the middle one joins together when it is whole, and it does not join together when it is pierced; and the outer one is pure [i.e. does not join together, and is considered to serve neither as a handle nor to preserve the food] in either case.

כָּל הַקְּלִפִּין מִטַּמְּאוֹת וּמְטַמְּאוֹת וּמִצְטָרְפוֹת. רַבִּי יְהוּדָה אוֹמֵר, שְׁלשָׁה קְלִפִּין בַּבָּצָל. הַפְּנִימִית, בֵּין שְׁלֵמָה בֵּין קְדוּרָה, מִצְטָרֶפֶת. הָאֶמְצָעִית, שְׁלֵמָה מִצְטָרֶפֶת, וּקְדוּרָה אֵינָהּ מִצְטָרֶפֶת. וְהַחִיצוֹנָה, בֵּין כָּךְ וּבֵין כָּךְ, טְהוֹרָה: