fruit growing on low plants, melon
蓏 centers on fruit that grows on low, vine-like plants rather than trees, especially melons and gourds.
蓏 combines 艹 (grass/plant) with 㼌, a doubled form related to 瓜 (melon/gourd), suggesting fruit that grows on low plants.
The grass top 艹 sits above a doubled melon shape 㼌, like melons growing on low vines among the grass.
For ラ, imagine a low vine producing a melon with a bright 'la' label on it: la -> ラ, and the melon is the fruit of the low plant.