Niddah, Chapter 6, Mishnah 7. This section of the chapter presents a run of general rules, each one shaped the same way: first a category that automatically brings a second obligation with it, and then the reverse formulation, which does not hold true. Here the Mishnah applies that pattern to two of the agricultural mitzvos: pei'ah and ma'asros.
The Mishnah states: "Kol shechayav bepei'ah chayav bema'asros." Pei'ah is the mitzvah of leaving over a corner of one's field at harvest time for the aniyim, the poor. Whatever crop is included in that obligation is automatically included in the obligation of ma'asros as well, meaning that a tenth must be separated from it. Once a crop carries the laws of pei'ah, the laws of tithing come along with it as a matter of course.
Now the second clause: "Veyeish shechayav bema'asros ve'eino chayav bepei'ah." Reversing the statement does not produce a true rule. Some crops are indeed obligated in separating a tenth, and yet the mitzvah of setting aside a portion at the edge of one's field for the poor has no application to them whatsoever.
This is the case with certain vegetables and certain fruits. They are fully chayav in ma'asros, and a tenth must be taken from them, but they are not chayav in pei'ah.
The result is a single principle stated from both sides: ma'asros is the wider obligation, and pei'ah is contained within it. Everything subject to pei'ah is subject to ma'asros, while a whole range of produce is subject to ma'asros without ever being subject to pei'ah.