sorry,but I didn't get the end , Professor is on of them in hana no migoro ni Arikawa or ...

This story goes way back before Misaki Shouta's parents were even born. This story is talking about the love journey of Shouta's grandpa and the professor Shouta was working for back in the university (Hana no Mizo Shiru). Because of some inevitable circumstances, the pair had to part ways. Shouta's grandpa got married to his grandma and took up her last name Misaki, and the professor ended up working at the university. At the end of the story, the old professor shown is Shouta's grandpa's lover and the boy is Shouta himself. The professor was glad to see Shouta because Shouta reminded him of his lover Akira (Shouta's grandpa) who was already dead by then. I hope my explanation makes sense for you lol

if I understand , in the end the couple from Hana no Mizo Shiru than not together eventually, and this couple also was separation in the end, so all couples in Hana Series were not together ind the end ( not happy ending for alle couples)?

I think you confuse the timing a little bit here. In the epilogue of this manga, It was Shouta's first time coming to the University (or his first time coming to the lab). This happened before Shouta met Arikawa --Think of this as the flashback before Hana no Mizo Shiru started--
The story in Hana no Mizo Shiru began after Shouta started working in the lab and his first encounter with Arikawa --Fast forwarding after the ending of Hana no Miyako de--

yes I was confuse, but now I understand, thanks a lot for your help ٩(๑❛ᴗ❛๑)۶

My pleasure!! (⌒▽⌒)