Sheviis, Chapter 1, Mishnah 7. In the previous mishnah we learned that someone who has ten saplings - ten young trees - scattered throughout a beis se'ah in his field may plow the entire field until Rosh Hashanah, and this ruling was transmitted as a halachah given to Moshe at Sinai. Our mishnah expands on that same principle and takes it one step further.
The words of the mishnah:
"HaNetios v'haDelu'in mitztarfin l'soch beis se'ah" - saplings of trees and delu'in (gourd plants) combine together to make up the number ten for this halachah of a beis se'ah. So, for example, six saplings and four gourd plants combine to make ten, and that is enough to be considered ten saplings and to permit plowing until Rosh Hashanah.
The early commentators explain, however, that this combination is effective only on condition that the number of saplings exceeds the number of gourds, and a ratio of at least six to four is required.
The view of Rabban Shimon ben Gamliel:
Rabban Shimon ben Gamliel takes the ruling one step further: "Kol eser delu'in l'beis se'ah - chorshin kol beis se'ah ad Rosh HaShanah" - even if there are no saplings there at all, but only ten gourd plants in a beis se'ah, one may plow the entire field, up to the area of a beis se'ah, until Rosh Hashanah.
The early commentators clarify that this is not referring to an ordinary gourd, but specifically to a Greek gourd, which was large and significant like the sapling of a tree.
In summary: in this mishnah we learned that saplings and gourds combine together to make up ten within a beis se'ah, provided that the saplings outnumber the gourds in a ratio of at least six to four; and according to Rabban Shimon ben Gamliel, ten gourds in a beis se'ah are enough - and these are the Greek gourd, which is considered like a sapling - to permit plowing the entire beis se'ah until Rosh Hashanah.