Flower: completed!

The tower was indeed the key to the sixth level, and indeed the whole game.  Winding up through the city, lightening the buildings and swings, I entered the mass of girders and destroyed as many as I could as I was blown ever upwards.  There were few flowers inside the structure, but as I reached the top I had a huge number of petals following me.  The tower fell, and a tree grew.  End of game.

I've found three hidden flowers on the first level, and one one every other level.  I'll be back to play this again.