Mishnah Uktzin 2:7: The green leaves of vegetables join together [to the vegetables to count towards the measure for impurity], and the white ones do not join together. Rabbi Elazar bar Tzaddok says: the white ones of cabbage join together, since they are edible; and so also of lettuces, since they preserve the edible part.

עֲלֵי יְרָקוֹת יְרֻקִּים, מִצְטָרְפִין. וּלְבָנִים, אֵינָן מִצְטָרְפִין. רַבִּי אֶלְעָזָר בַּר צָדוֹק אוֹמֵר, הַלְּבָנִים מִצְטָרְפִים בַּכְּרוּב, מִפְּנֵי שֶׁהֵן אֹכֶל. וּבַחֲזָרִים, מִפְּנֵי שֶׁהֵן מְשַׁמְּרִין אֶת הָאֹכֶל:

Mishnah Uktzin 2:8: Regarding onion leaves and the offshoots of onions, if there is moisture within them they are measured as they are [for matters of impurity]; if they contain hollow spaces within them, one should flatten their hollows [and then measure them]. Spongy bread is measured as it is; but if it contains any hollows, one must flatten its hollows [and then measure it]. The flesh of a calf which has expanded [during cooking], or the flesh of an old [beast] which has shrunken, are measured as they are.

עֲלֵי בְצָלִים וּבְנֵי בְצָלִים, אִם יֵשׁ בָּהֶן רִיר, מִשְׁתַּעֲרִין בִּכְמוֹת שֶׁהֵן. אִם יֵשׁ בָּהֶן חָלָל, מְמַעֵךְ אֶת חֲלָלָן. פַּת סְפוֹגָנִית, מִשְׁתַּעֶרֶת בִּכְמוֹת שֶׁהִיא. אִם יֶשׁ בָּהּ חָלָל, מְמַעֵךְ אֶת חֲלָלָהּ. בְּשַׂר הָעֵגֶל שֶׁנִּתְפַּח וּבְשַׂר זְקֵנָה שֶׁנִּתְמַעֵט, מִשְׁתַּעֲרִין בִּכְמוֹת שֶׁהֵן: