Peah, Chapter 3, Mishnah 3. This mishnah distinguishes between two forms of the very same species of plant, and teaches that in certain cases they are viewed as two separate obligations of peah.
Moist onions and dry onions:
The mishnah discusses one who "shemachlik" - a term meaning to pull up and uproot from the ground - moist onions, meaning fresh ones, in order to sell them in the market, while leaving other onions to remain less moist and dry out further, and those he uproots at the time of the granary, when all the produce is brought in. In such a case he gives peah for each group on its own.
Even though both types are the same species of onion, since one of them he pulls up early and uses for an occasional sale in the market, while the other he leaves until it fully ripens, they appear as two different species with regard to peah.
The same law applies to legumes, and likewise in a vineyard, to grapes.
Thinning the vineyard:
The mishnah now discusses one who performs something resembling pruning. Pruning is the cutting of a tree's branches for the tree's own benefit; and here the case is thinning a vineyard, removing some of the vines so that the remaining vines can grow better. The vines he thinned out he has in effect harvested early, since the grapes on them are fit for use or for sale sooner, while the others he preserves until they ripen completely.
In this case peah is taken only from what remains, and this peah counts only for what remains. As for what was thinned out earlier, it carries no obligation of peah at all.
The reason is this: the purpose for which the thinning was done was to benefit the vines that remain, and this is not considered harvesting - and the Torah requires harvesting in order to obligate peah.
Uprooting from one place:
However, if instead of thinning the vineyard in various spots he uprooted all the vines from one place - and the term "machlik" means to smooth, to make that entire area smooth - then he has not thinned in a general way, but has cleared out one place. Here too he takes peah from what was left behind and is still in the ground, but this peah exempts the entire vineyard, even the portion he removed at the outset.
The reason for this: such uprooting already has the status of harvesting. Since he took all the vines together from one place, it is clear that he is not removing them merely to prune and benefit what is left behind, but is harvesting them as a group. And since this is considered harvesting, it too is obligated in peah.
In summary: This mishnah taught that two forms of the same species - moist onions uprooted for the market and dry onions kept until the granary, and likewise with legumes and in a vineyard - are treated as two species with regard to peah, and each one is obligated in peah on its own. We also saw the distinction within the vineyard: one who thins his vines in various spots in order to benefit those that remain has not performed an act of harvesting, and therefore gives from what is left over for what is left over alone; whereas one who uproots all the vines from a single place has performed an act of harvesting, and the peah he gives from what remains exempts everything.