seed, pip, pit, stone
籽 centers on the small, hard reproductive part of a plant: a seed, pip, pit, or stone. All meanings are concrete botanical parts from which new growth can emerge.
籽 combines 米 (rice, grain) with 子 (child, seed), suggesting a grain-like seed or the offspring of a plant. The exact historical development is uncertain, but the component choice fits the meaning well.
Rice 米 and child 子 together suggest a tiny grain that can grow into a new plant: a seed, pip, or pit.
For シ, imagine a seed (籽) that looks exactly like the letter C: C -> シ, and you plant that C-shaped seed in the ground.
—