You dig your toes into the ground, feeling odd sensations of pleasure wash over you. You shift your bare feet around, feeling the earth beneath them. When the sensations become too intense, you shed your clothing, tossing it aside, and begin stroking your member. You feel the sun beating down on you, you want to soak it in, you want to be close to it. Suddenly, your feet feel stiff and your toes become unresponsive.
Looking down, you watch as your feet become rough to the touch, eventually turning brown, all the while stroking yourself. Your feet grow together, the rough texture creeping up your legs, just a few inches before the bark. Wherever the bark touched, your body swelled to massive proportions and became numb. Before long, you reach an orgasm and begin spraying cum. However, what's coming out of your penis is not cum, it's thicker and sticky. You notice it has become tree sap. You see your legs are gone and the rough texture has reached your testicles. Suddenly, you feel your balls quiver as the stream of tree sap stops. They recede into your trunk, just under your penis.
You frown slightly as the rough texture reaches your penis. You still attempt to stroke it even as it grows outward. Soon it doesn't even remotely resemble its former self. It's three feet long and covered in small leafs and tree bark. Suddenly, you feel a massive sensation of pleasure coming from the tip of your new branch. Apparently the head of you penis remained intact, albeit a bit swollen. You continue to watch in fascination as it swells further, turning green, and bursts to reveal a white flower.
The rough texture continues up your body followed by tree bark and swelling. More branches sprout from various places as your body grows numb. Your arms are quickly overtaken by the changes as they stiffen and split into countless branches. You begin to feel sleepy as your facial features disappear. You resist the urge to close your eyes for a moment but you just feel so sleepy. You close your eyes for the last time and your head explodes into numerous branches and leafs.
You will never change back. You remain planted in this spot for decades, even beyond the life of Jack, until you succumb to time and whither away.