(seed) capsule
蒴 refers specifically to a seed capsule, the dry fruit structure that encloses seeds in plants like poppies or irises.
蒴 combines the grass radical 艹 with 朔, which likely provides the sound. The character is used in botanical contexts for seed capsules.
The grass radical 艹 sits above 朔, like a plant whose seed capsule forms at the top after flowering.
For サク, imagine a sack full of seed capsules: sack -> サク, and the grass top 艹 holds the capsule inside.