The hatching on your leaves entirely neglects the curvature of the leaf itself - they're just straight lines, being drawn across a somewhat curving surface, so the hatching is flattening it out.

The lines you see on real leaves are veins - that's an actual texture, and therefore if you want to try and capture them, then you would be doing so using textural techniques as described back in lesson 2 (capturing the shadows they cast to imply the presence of those textural forms, not just drawing them as simple lines). Otherwise, for now, you can just focus on them behaving as simple contour lines. Leaves are probably the only place where you can get away with adding a bunch.

As to your last image, no - each of those lines define the 'flow' of the individual, smaller leaf form, and therefore they each serve a clear purpose. They are not extraneous in any way. There you're following the same principles laid out in the correct form of this section.

All that matters is that you weigh whether or not the mark you're looking to draw serves a purpose within your drawing that has not yet been accomplished by any other. If it does, then draw it. If it does not, don't. The planning phase of the ghosting method is ideal for determining whether or not the mark is necessary.