Kommentar zu Kilayim 1:5

The radish and the cabbage turnip, mustard and charlock mustard, Greek gourd with Egyptian gourd or [Greek gourd] with bitter gourd, even though they are similar one to the other, are nevertheless kilayim one with the other.
This mishnah returns to discussing kilayim in vegetables. In the earlier mishnayot (1-3) the mishnah listed vegetables that were similar enough not to be kilayim one with the other, and therefore permitted to plant together. In today’s mishnah we learn of vegetables that are similar but not similar enough and therefore are kilayim one with the other.
Note that the first five mishnayot form a chiastic structure, meaning an x-like literary structure (usually a-b-b-a, or perhaps a-b-c-b-a). We began with vegetables that are not kilayim (A1), continued with trees that are not kilayim (B1), then trees that are kilayim (B2) and finally vegetables that are kilayim (A2). This is a typical structure in rabbinic literature.