Midrash su Levitico 11:37

1) (Vayikra 11:37) ("And if there fall of their carcass on any variety of seed which is sown, what is sown, it is clean.") "And if there fall of their carcass": There is of their carcass which imparts tumah, and there is of their carcass which does not impart tumah. This excludes a dried up carcass, which cannot (return to its moistness) by soaking.

2) Whence is it derived that seeds which were tamei, which were sown, are clean? From "what is sown it is clean." I might think that this is so even if it did not take root; it is, therefore, written "it is" (i.e., if it did not take root, it remains in its original state of tumah.)

3) "And if there fall of their carcass on any variety of seed which is sown, what is sown, it is clean. (Vayikra 11:38): And if water be placed upon a seed, and aught of their carcass fall on it, it is unclean to you.": There are in this (formulation numerous variables): seeds that are tamei and seeds that are clean, rooted to the ground and uprooted from the ground, placing (of water on the seed) by man and placing by Heaven, (seed for) human consumption and (seed for) animal consumption. Distinctions must be made. If you say the rooted are tamei and the uprooted are clean, you have made everything unclean! (for everything was originally rooted.) And if you say the rooted are clean and the uprooted are tamei, you have made a part tamei and a part clean.