The Torah instructs in (Exodus 12:22), "And you shall take a bunch of hyssop," referring to the bundle of hyssop used to apply the blood of the Paschal lamb to the doorposts in Egypt. The Mekhilta extracts from this single verse a sweeping principle about ritual practice throughout the entire Torah.
The reasoning works by analogy. Across the Torah, many commandments require "taking" something — taking a lulav, taking cedar wood for purification, taking oil for anointing. In most of these cases, the Torah does not specify how the item should be gathered or held. The instructions are "unqualified" — they simply say "take."
But here, with the hyssop, the Torah qualifies the taking: you must take a "bunch" — an agudah, a bundle bound together. Since this is the one instance where the Torah specifies bundling, the Mekhilta derives that all "takings" in the Torah should follow the same pattern. Wherever the Torah commands you to take something for a ritual purpose, you should gather it in a bunch.
The Mekhilta then turns to the hyssop itself with exacting precision. Not just any plant called "hyssop" qualifies. Greek hyssop is excluded. Roman hyssop is excluded. Cochalite hyssop is excluded. Desert hyssop is excluded. The rule is absolute: any hyssop that requires a qualifying adjective — any variety identified by a regional or descriptive epithet — is disqualified. Only plain, unmodified hyssop counts.
The simplest version of the plant, like the simplest reading of the law, is the one that stands.