I'm curious whether the shorter stroke or longer stroke are the extra strokes added to 木 to form the characters 未 and 末? And do these strokes have a semantic significance? I learned that 本 has the extra stroke at the bottom for the meaning of root. I'm expecting that 未 and 末 have a comparable explanation that might help me remember them, as I'm continually getting these two mixed up.
末 is logographically comparable to 本. The top stroke indicates the top branches of a tree, the end of the tree. Hence the meaning "end".
However, 未 is unrelated to both characters above, it is just a representation of a tree. It is one of the twelve earthly branches, and as such a character that once had some original meaning that is now lost to us; it has been abstracted to mean "not yet".
「未」depicts a flourishing tree (tree with more branches or longer branches than「木」), indicating the meaning fragrance (i.e. when trees/plants are in full bloom). The current shape is derived from a variant with two more branches drawn, and these extra branches became straightened out into one stroke over time.
This word that「未」represented is now written as「味」(e.g. 味道, smell), while the senses not yet (e.g. 未來, future) and earthly branches (地支) are both phonetic loans.
「末」depicts a tree with a mark at the upper end, indicating the meaning upper extremity > end. The mark grew in length over time.