> Sometimes, as I sat at my window, so heedlessly did they (boughs) grow and tax
> their weak joints, I heard a fresh and tender bough suddenly fall like
> a fan to the ground, when there was not a breath of air stirring,
> broken off by its own weight.

 1. I assumed that the complete sentence containing 'broken off by its own weight' is 'I heard a fresh and tender bough broken off by its own weight', and the 'when' phrase is a parenthesis.

  But it seems like that the verb after a verb of perception should be in the simple form or -ing form, Can I use past tense verbs here? Or did I misunderstand the sentence? 


 2. I figure 'stirring' is used to describe 'a breath of air', can a gerund be used like this? Thank you very much.