Peah, Chapter 1, Mishnah 4. This mishnah teaches us the conditions that must be met for a particular crop to be obligated in peah. We will go through each condition one by one, and explain what is included in it and what is excluded by it.
"Klal amru b'peah":
The mishnah lays down a general rule by which one can determine what is obligated in peah and what is exempt from it. These are the conditions:
"Kol shehu ochel" - something that is considered food. This condition comes to exclude types of plants grown for the purpose of dyeing. Even though a person may sometimes eat them out of need, since their primary use is for dye and not for eating, they are not obligated in peah.
"V'nishmar" - something that the owner takes care to guard, as opposed to ownerless produce. Something that is not in a person's ownership is not obligated in peah.
"U'giduluv min ha'aretz" - that it grows from the ground. This condition comes to exclude truffles and mushrooms.
"U'likitato k'echad" - that it is gathered all at one time. There are fruits and crops that are harvested all at once, and there are those that are not. An example of this is figs, which ripen little by little and are not gathered all at once, and therefore they are not obligated in peah.
"U'machniso l'kiyum" - that it be something a person stores away and keeps. Leafy vegetables are excluded from this rule, since they wilt immediately and cannot be placed in storage.
The mishnah concludes with examples of things that are obligated in peah: grain and legumes. The grains, as well as the legumes and beans, are all included in the rule stated regarding peah, and therefore they are all obligated in peah.
In summary: In this mishnah we learned five conditions for the obligation of peah - that it be food, that it be guarded, that it grow from the ground, that it be gathered all at once, and that it be stored away for keeping. These conditions exclude dye plants, ownerless produce, truffles and mushrooms, figs and leafy vegetables; grain and legumes, however, are included in the rule and are obligated in peah.