Hello,
i can highly recommend you to use the Threshold Colour plugin in your special case.
http://imagejdocu.tudor.lu/imagej-documentation-wiki/pluginsWith your image it works good in the HSB and CIE Lab colour space to work out the leaves.
If this is sufficient for your work you can for example threshold the image in this dialog.
Then convert the image to 8-bit (
Image->Type->8-bit) and threshold the black areas with
Image->Adjust->Threshold (will be coloured red).
You can now make a particle analysis
Analyse->Analyse Particles (Set the values you want to measure
in the "Set Measurements" dialog). In the "Analyse Particles" dialog you can select
at which size the particles should be counted (because there will be a
little bit "noise" in your image).
Before you do this please improve the quality of your image with imageJ or a standard image
tool like Gimp etc. to work out the colours!
I hope this information helps
With kind regards
Marcel