branch
朶 centers on a branch of a tree, specifically a drooping or hanging branch. It is a rare kanji used mainly in literary contexts for this image.
朶 combines 乃 (a pictograph of a curved object) with 木 (tree), suggesting a drooping branch. The exact historical development is uncertain, but the component combination visually conveys the meaning.
The top 乃 looks like a curved, hanging shape, and 木 is a tree. Picture a tree with a branch that curves downward under the weight of fruit or leaves.
For ダ, imagine a branch so heavy it makes a dull 'duh' sound as it droops: duh -> ダ. The branch hangs low, just like the sound.