Kilayim, Chapter Three, Mishnah Four
This mishnah deals with a person who wishes to plant several types of vegetables within one field.
1) Planting two rows of cucumbers, two rows of gourds, and two rows of Egyptian beans is permitted.
2) [But planting] one row of cucumbers, one row of gourds and one row of egyptian beans is prohibited.
3) [Planting] one row of cucumbers, one row of gourds, one row of Egyptian beans and [again] one row of cucumbers:
a) Rabbi Eliezer permits,
b) But the sages forbid.
Section one: When one plants two rows of cucumbers it looks like a separate field. Therefore, one can plant next to it two more rows of gourds and then two more rows of Egyptian beans. There is no need for any minimum length of the rows, as there was in yesterdays mishnah, because each row looks like its own field.
Section two: However, if one plants a field with a row each of three different species, it does look like kilayim, and it is prohibited.
Section three: In this case he plants four rows of three different vegetables. Somewhat surprisingly, Rabbi Eliezer permits this. Albeck explains that according to Rabbi Eliezer, in this case it will be noticeable that he is not simply mixing up species, but planting alternate rows of three species, cucumbers, gourds and beans. Since it doesnt look like kilayim, Rabbi Eliezer allows this arrangement. The other sages however still forbid this.