Parah, Chapter 11, Mishnah 7. The latter part of this chapter turns to the eizov, the hyssop plant that is bound and dipped into the mei chatas and used to sprinkle a tamei person or vessel. The Torah simply says eizov, and from that single word the Mishnah derives a series of rules: which plants qualify, which are disqualified, and which parts of the plant may be used.
Eizov and eizov with a second name
The Mishnah lays down the governing principle: "Kol eizov sheyeish lo sheim levai, pasul." Any hyssop that carries an accompanying name, a qualifier attached to the word eizov, is unfit for sprinkling. "Eizov zeh, kasher": plain hyssop, the plant whose name is simply eizov and nothing more, is fit. The Torah asked for eizov, not for a variety of eizov.
Illustrations follow. "Eizovyon" is a hyssop employed as a remedy for worms in the intestines. There is also the blue-tinted sort, "eizov kochali"; the Roman kind, "eizov romi"; and the variety that grows in the wilderness, "eizov midbari". Of all of them the Mishnah rules "pasul". Each one is a specific breed of hyssop rather than hyssop plain and simple, so none of them is acceptable for use with the mei chatas.
Hyssop that is terumah
"Veshel terumah temeiah, pasul." Hyssop that has the status of terumah and has become tamei is disqualified for sprinkling. "Veshel tehorah, lo yazei, ve'im hizah, kasher." Hyssop of terumah that is still tahor should not be used to sprinkle lechatchilah, but if one went ahead and sprinkled with it, the sprinkling is valid after the fact.
Which part of the plant
"Ein mazin lo beyonkos velo bitmaros." One does not sprinkle with the yonkos, the young shoots, nor with the temaros. What are temaros? The most widely accepted explanation, that of the Rosh and the Bartenura, is that these are the berries of the plant. The Tosefta suggests a different reading, that they are soft branches which have not yet blossomed. We follow the standard understanding of berries.
Entering the Mikdash after such a sprinkling
Since a sprinkling done with yonkos is invalid, one who was sprinkled that way remains tamei. Still, the Mishnah teaches: "Ve'ein chayavin al hayonkos al bi'as hamikdash." One is not held liable for entering the Mikdash after having been sprinkled with young shoots. Practically speaking, such a person is not treated as one who entered the Sanctuary in a state of tumah.
Rebbi Eliezer extends this: "Af al hatmaros", the same leniency applies to one who was sprinkled with the berries. The halachah, however, is not decided in accordance with Rebbi Eliezer.
Finally the Mishnah defines its term: "Eilu hein hayonkos: givo'lin shelo gamlu." The young shoots are the stalks that have not yet ripened.
The thread running through the entire Mishnah is precision. The mei chatas is sprinkled with eizov, and the Torah's word is taken exactly as it stands: not a named variety, not an unripe stalk, not a berry, but the hyssop itself.