Which is the additional stroke added to 末 and to 未? significance? 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".