castor-oil plant
蓖 refers specifically to the castor-oil plant, a plant known for its large leaves and oil-rich seeds.
蓖 is a phono-semantic compound: the plant radical 艹 indicates a plant, while the lower part (比 over 囗) likely contributes the sound. The exact historical development of the lower component is uncertain.
The plant radical 艹 sits above a shape that looks like a container (囗) with two spoons (比) inside. Imagine a castor-oil plant growing over a pot of its own oil.
For ヒ, picture a castor-oil plant with a large leaf that looks like a heat shield: heat -> ヒ, and the plant's oil is used in many heated processes.