yarrow, sericea, stalks used for divination
蓍 centers on the yarrow plant, specifically its use in traditional divination where its stalks are cast to interpret patterns. The meaning extends naturally to the plant itself and the divination stalks.
蓍 combines the plant radical 艹 with 耆, which likely provides the sound and may relate to the idea of old or experienced, fitting a plant used in ancient divination.
The top 艹 shows it is a plant, and the bottom 耆 looks like an old sage with a sun 日 and a staff-like shape. Picture an old sage using this plant's stalks under the sun to divine the future.
For シ, imagine a sheaf of yarrow stalks being shuffled for divination: sheaf -> シ, and the sound of the stalks rustling helps you remember the reading.