Matt
Thanks for sharing these.
#1 - excellent, would be just a notch better with no shadow top left IMHO
#2 - doesn't work for me for a number of reasons. Biggest for me is the diagonal line created by the centre of the leaf which breaks the image oddly. Probably breaks one of those "immutable" laws of composition too

I also find the brown rust marks distracting. The real shot here for me would be a eggstream close up of the centre where you can actually see the cellular structure.
#3 - whereas the line leads my eye out of the shot in #2 it has the opposite effect. This is my pick of these. The image is made stronger by the impact of the dhadows since they have a clearly defined shape (as opposed to #1 where the shadow is a blurry distraction)
#4 - Anothe rexcellent use of light and dhade, just behind #3 in my view.
#5 - doesn't work for me. The high key lighting in the background distracts me too much from an examination of the interesting symmetry of the leaf structure.
Edit: I'd like to see a B & W of #5