I though I named the author and even the story title, seems like I edited and deleted everything about it, yes it is from Youkihi, there was also a story titled Eien no Shounen (Eternal Boy), it gives the impression to have AP but is just the story forwarding years into the future while the boy doesn't age, and also another titled Powered Girl.
[Youkihi(陽気婢)] Asoko Kinoko 1 & 2 [1991-05]
Two chapters, compiled at least two times:
http://www.megaupload.com/?d=JH4OYRPI [from: Magic Mushroom (マジック・マッシュルーム)[1992-07-05]
http://www.megaupload.com/?d=VJBG99P5 [from: Gira Gira (いつもギラギラする日)][1995-02-28(1995-03-20)]
I believe that's everything there is to the story, there are other mushroom stories but not related to this.