Maseches Shevi'is, Chapter 1, Mishnah 6. In Mishnah 4 we learned that in an orchard - a field of trees - the requirement that each individual tree be capable of producing a certain quantity of fruit applies only when there are fewer than ten trees; but once there are ten trees, they need not each produce that quantity. Our Mishnah deals with a different calculation that also involves the number ten.
"Eser neti'os mefuzaros besoch beis se'ah - chorshin kol beis se'ah bishvilan ad Rosh Hashanah":
The Mishnah is speaking about ten neti'os - that is, very young trees, saplings, and not mature trees. The precise definition of a 'netiah' will be explained below in Mishnah 8.
Regarding these ten saplings, when they are "mefuzaros besoch beis se'ah" - scattered throughout the entire area of a beis se'ah - an exceptional halachah was stated: one may plow the whole beis se'ah on their account, and not only until Shavuos as mentioned earlier, but all the way until Rosh Hashanah.
Chazal taught us that this leniency is a halachah l'Moshe miSinai, and therefore it applied even at the time when Shemittah was in force by Torah law. This is an exception to the rule: although normally tosefes shevi'is is required - adding on to the Sabbatical year - for ten saplings this requirement did not apply.
"Hayu asuyos shurah o mukafos atarah":
All of the above was said specifically when the saplings are scattered throughout the beis se'ah. But if they are planted in a straight line, or they are "mukafos atarah", the law is different. Two explanations are offered for the expression "mukafos atarah":
The saplings are not scattered but are enclosed and gathered together within a fence of their own.
The saplings are planted in the shape of a crown, like an arc - that is, in a semicircle.
In such a case, when they stand in a row or in a semicircle, "ein chorshin lahem ela l'tzorkan" - there is no permission to plow the entire beis se'ah, since the saplings are not spread out over all of it, but only the area that they actually need. The measure of that area is as explained in the previous mishnayos: its full space, or the space of the picker and his basket - enough room around the tree for the person harvesting the fruit together with his basket. In that area one may plow until Rosh Hashanah, while the rest of the field must be treated as a white field, a field of grain, and therefore may be plowed only until Pesach.
In summary: In this Mishnah we learned that ten saplings scattered throughout a beis se'ah permit plowing the entire beis se'ah until Rosh Hashanah, and this is a halachah l'Moshe miSinai that applied even when Shevi'is was in force by Torah law, and for which tosefes shevi'is was not required. But when the saplings stand in a row or are mukafos atarah - whether they are enclosed by a fence or planted in a semicircle - one plows only for their needs: the area around them until Rosh Hashanah, and the rest of the field follows the law of a white field, until Pesach.