The large purple plant likes light, but you need to close it up.
After you go left from the junction of whether you could go left or straight, turn right. Go all the way down the path. You should now be standing in front of a tall, green plant. As a general rule for this game, if you see something that you can touch, click on it.
Since you need to close up the large purple plant, click on the tall green plant so it will block out the sunlight.
Oops - I need to get back up to the big tree. How do I do that?
Can you explain what I'm actually trying to do on this level?