The round adhesive electrodes are reusable and can be used on strong leaves. When you don't get to the leaves of a tree, you can use them on the stem, but be careful not to put it on bark that is too brittle, otherwise you will soil it irretrievably. Clip electrodes are ideal for plants with leaves that are too small or delicate, for fungi, or for humans. We recommend wetting the surface of the leaves where you apply them for better detection, whereas the round adhesive sensors already have gel incorporated.