Uktzin 1:1

If a part of produce serves as a handle but doesn’t protect it, it both contracts and conveys ritual impurity but it doesn’t combine to make up the volume. If it protects but isn’t a handle, it contracts and conveys impurity and it does combine. If it neither protects nor serves as a handle, it doesn’t contract or convey impurity.

Uktzin 1:2

Protective parts of produce include roots of garlic, onion and leeks that are still moist; their blossom ends whether moist or dry; the central stalk that’s close to edible part; the roots of lettuce and of long radishes and round radishes; this is the opinion of Rabbi Meir. Rabbi Yehuda says that only the large roots of long radishes combine; their fibrous roots don’t. The roots of mint, rue, wild herbs and garden herbs that have been uprooted for transplanting, the spine of an ear of grain and its husk; Rabbi Elazar adds the dusty covering of root vegetables: all of these things contract and convey ritual impurity and combine towards the volume.