A sheath is the outer cover around the male and female flowers of a date palm. It has a woody texture to protect the flowers and then carry them after they stem out of it. Female flowers grow into clusters carrying green and ripe palm fruits. When those clusters dry, they turn into stalks.
Allah, Glory is to Him, says: To him [alone] is attributed knowledge of the Hour. And fruits emerge not from their coverings nor does a female conceive or give birth except with His knowledge. And the Day He will call to them, “Where are My ‘partners’?” they will say, “We announce to You that there is [no longer] among us any witness [to that].” [Fussilat: 47]