grassy plant with fibres from which cloth is made
檾 refers to a specific grassy plant whose fibers are used to make cloth. The meaning is narrow and technical, with no clear conceptual center beyond the plant itself.
檾 is a rare character. The modern form visually contains 林 at the bottom with a complex upper part that includes 冖 and 火, but the exact historical development is uncertain. It likely originated as a phono-semantic compound related to the plant name.
The bottom 林 suggests many plants growing together. The upper part with 冖 and 火 might be imagined as a covering and heat used to process the plant fibers into cloth. Picture a dense grove of this grassy plant being harvested and treated to make fabric.
For ケイ, imagine a key (ケイ) plant that unlocks the secret to making strong cloth. The key-shaped leaf of this grassy plant is the clue: key -> ケイ.