luxuriant vegetation
薈 centers on thick, flourishing plant growth: a scene of vegetation so dense and healthy it feels almost excessive.
薈 combines 艹 (grass/plant) with 會, which likely contributes the sound and possibly the idea of gathering or abundance, suggesting plants growing thickly together.
The grass top 艹 over 會 (a shape associated with gathering) paints a picture of plants meeting and crowding together into a lush, overgrown tangle.
For カイ, imagine a thick carpet of vegetation so dense it looks like a giant green pie: pie -> カイ, and the plants are luxuriant enough to cover everything.
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aloe