Old English fleax "flax plant; cloth made with flax, linen," from Proto-Germanic *flakhsan (source also of Old Frisian flax, Middle Dutch and Dutch vlas, Old Saxon flas, Old High German flahs, German Flachs), probably from Proto-Germanic base *fleh- "to plait," from PIE root *plek- "to plait." But some connect it with PIE *pleik- (see flay) from the notion of "stripping" fiber to prepare it.
Old English sed, sæd "that which may be sown; an individual grain of seed; offspring, posterity," from Proto-Germanic *sediz "seed" (source also of Old Norse sað, Old Saxon sad, Old Frisian sed, Middle Dutch saet, Old High German sat, German Saat), from PIE *se-ti- "sowing," from root *sē- "to sow." Figurative use in Old English. Meaning "offspring, progeny" rare now except in biblical use. Meaning "semen" is from c. 1300. For sporting sense, see seed (v.).