Commentaire sur Le Lévitique 11:37

זרע זרוע means seed of any kind of seed-plant. The word זרוע is a noun (not a passive participle, when it would be punctuated ‎זָרוע), like (Daniel 1:12) "and let them give us of pulse — זֵרוֹעִים".

ZERA ZEIRUA’ (SEED OF SOWING). “Zera means ‘seed’ of any kind of seed-plant. Zeirua is the noun (‘sowing,’ ‘thing sown’), like the expression, and let them give us ‘zeiro’im’ (edible seeds) to eat.²⁶⁶Daniel 1:12. [And if aught of their carcass fall upon any seed of sowing which is to be sown] it is clean. This is to teach you that food is not fit and susceptible of becoming impure until water has once come upon it.” This is Rashi’s language.
Now Scripture mentioned above, of all the food which may be eaten, that on which water cometh [shall be unclean],²⁶⁷Verse 34. establishing the principle that foodstuff must first become susceptible to impurity [through coming into contact with water]. But there the Rabbis interpreted it to mean²⁶⁸Mentioned in Rashi ibid., on basis of Torath Kohanim. the foodstuff is not rendered impure through the medium of the interior of an [impure] earthen vessel, unless it was made susceptible of receiving impurity by being moistened by water. But here Scripture added that even from [contact with] the creeping things themselves, foodstuff does not become impure unless it was made susceptible [by water]. So it is explained in the Torath Kohanim.²⁶⁹Torath Kohanim, Shemini 11:8.
It is also possible to say that the expression of all the food which may be eaten.²⁶⁷Verse 34. refers to the things mentioned above — meat and various foods. But here He states that even seeds when taken out to be sown, can be rendered impure if they have become susceptible to impurity by being moistened by water. The reason for foodstuffs having to be made susceptible by water [before they can become impure], is that the uncleanness of [dead] creeping things and the other sources of impurity, attaches to foodstuffs in their state of moisteness, but not when they are dry. The Torah then, as an extra precautionary measure, declared impure those foods on which water had once come, although they have [subsequently] become dry, [if they were then touched by any of the sources of impurity], in order that the rule should not vary according to different standards. Now the law of making food susceptible of becoming impure applies also to the impurity [conveyed] by carrion, Scripture having mentioned it here for the reason we have stated above.²⁷⁰See Ramban at end of Verse 36 above.

אשר יזרע (בקרקע) טהור. Anything still connected to the earth is not subject to ritual defilement.