On Batman Arkham Asylum, I'm stuck?

After the batcave fills up with Poison Ivy's plants, they want me to escape the batcave and destroy Poison Ivy's plants. Well, I was stuck so I went back in the back in the batcave all the way to the sewer sysetem's overhang and glided off to a grass beam where I pulled a ventilation shaft off the wall to find I couldn't get escape so I went back through the shaft and tried to grapple back to the overhang. It wouldn't let me. I tried the line grappler, and when I went through the door on the grass beam, it was covered by plants. Help!