(Mishnah): Any Seah of seeds (to be planted) that has in it a Beis Rova (quarter Kav) (i.e. 1/24th) of another seed, he must reduce it (to less than 1/24th - either by increasing the majority or reducing the minority).

(R. Yosi): He should remove all of the quarter Kav, whether it contains one or two species.

(R. Shimon): The prohibition of a Beis Rova (quarter Kav) of another seed is only if it is entirely from one type of seed (as if the quarter Kav consists of multiple species of seed, it is not prohibited).

(Chachamim): Anything which is Kilayim with the Seah's seed combines to make a quarter Kav. (Two types of seeds combine to make the quarter Kav only if both of them are considered Kilayim with the species of the Seah.)

When must a quarter Kav be reduced? When grain is mixed with grain (e.g. wheat with barley); beans with beans (e.g. beans and lentils) and beans with grain.

In truth they said that garden seeds that are not eaten (such as garlic or turnip seeds) combine to be 1/24th in a Beis Seah.

(R. Shimon): Just as they gave the rule of 1/24th resulting in a stringency (for garden seeds of which a small amount is sown in a Beis Seah), so too they gave it to result in a leniency (for seeds of which a large amount is sown in a Beis Seah)...

(Therefore) flax (that needs 3 Seah to seed an area of a Beis seah) that was mixed with grain, would also follow the rule of 1/24th.

(Gemara): Our text of the Mishnah is that it's a Seah 'that has in it' a quarter Kav of another species. Some Tannaim have the text 'that fell in it' a quarter Kav of another species.

(R. Mana): The text of 'has in it' understood that there were 23 quarter Kavs of one species and one quarter Kav of a second species, totalling 24 quarter Kavs. The text of 'fell into it' understood that there were already 24 quarter Kavs before the quarter Kav of a second species fell in, totalling 25 quarter Kavs.

(R. Yosi bei R. Bun disagrees): According to either text, the total was 24 quarter Kavs.

If so, what's the difference between the two texts? The text of 'has in it' refers to when the second species mixed in before they were planted. The text of 'fell into it' refers to the second species growing together with it.

What's the case? If he intended to plant the second species together with it, even if it was only one wheat stalk, it would be prohibited. If he mixes them to sell, no amount of a different species may be mixed (as he is defrauding the customer).