Shabbos, Chapter 12, Mishnah 2. This mishnah deals with labors performed in a field and on a tree, and with the minimum amounts that create liability for each one.
The four labors that open the mishnah:
"HaChoresh" - one who plows a field, since through his plowing he improves the field.
"HaMenakesh" - one who weeds the field and removes the grasses from it.
"VehaMekarsem" - one who cuts the dry branches off a tree.
"VehaMezared" - one who removes the fresher branches from a tree, pruning the tree in order to improve it.
In all of these the measure is "kol shehu" - one is liable even for the smallest amount. It depends on the scenario: either his liability comes because he improves the field, or because of sowing, improving the growth of a living thing, of the tree.
"HaMelaket etzim" (one who gathers wood):
"Im letaken" - if he gathers the wood from the field in order to improve it, so that there should not be branches scattered about in it, his measure is "kol shehu", since in this case his liability is because of plowing, because of improving the field, whose measure is even the smallest amount.
"Im lehasek" - if his purpose in gathering the wood is for kindling, that he wants to use it as fuel, his measure is "kedei levashel beitzah kalah": an amount of fuel sufficient to cook an egg that cooks easily, which is a chicken's egg, and in the volume of a dried fig. That is the significant amount for which he becomes liable, since he gathered the wood for fuel.
"HaMelaket asavim" (one who gathers grasses):
"Im letaken" - if he gathers ordinary grasses in the field in order to improve the field itself, his measure is "kol shehu", for this too is a form of plowing, whose measure is any amount at all.
"Im livheimah" - if he gathers them in order to feed them to an animal, his measure is "kimlo pi hagedi", an amount sufficient to fill the mouth of a young goat, as we have already learned in this tractate that this is the measure for the eating of grass.
What is the liability, in terms of the labors of Shabbos, in this gathering?
The liability is because of gathering sheaves (me'amer) - collecting items from the place where they grew and concentrating the produce together in one place. That is the essence of me'amer, and the measure for liability in it is kimlo pi hagedi, an amount sufficient to fill the mouth of a young goat.
In summary: In this mishnah we learned that one who plows, weeds, trims dry branches or prunes fresh ones is liable for any amount at all, whether because of improving the field or because of sowing. Regarding one who gathers wood and one who gathers grasses, everything depends on his intention: if it is to improve the field, his measure is any amount at all, because of plowing; if it is for kindling, enough to cook an easily cooked egg, in the volume of a dried fig; and if it is to feed an animal, enough to fill the mouth of a young goat, and his liability is because of me'amer.