You need to make a scatter brush
Important notes.
Have the leaf section selected
Go to Window>Brushes
With your object selected choose the 3 lines at teh top of the panel and new brush
Choose Scatter Brush
Create the Scatter Brush by saying OK - don't change anything yet
Draw a basic circle and then choose the Scatter Brush you created
Apply your Scatter brush (with your circle selected)
and then edit it by double clicking.
Adjust your settings until you're happy
