smilax china
葜 refers specifically to the plant Smilax china, a type of climbing vine. Its meaning is a single botanical name without broader abstract extensions.
葜 is a phono-semantic compound: the grass radical 艹 indicates a plant, and 契 likely provides the sound カツ. The exact historical development is uncertain.
The grass top 艹 shows it's a plant, and the bottom 契 looks like a tangled vine. Picture a smilax vine twisting like the strokes of 契 under the grass.
For カツ, imagine cutting (katsu) the tough smilax vine with a knife: katsu -> カツ, and the vine is finally trimmed.