You've demonstrated a considerable amount of care and patience throughout the completion of this challenge, and overall you've done a pretty great job with it. There are some issues I do want to point out that should help you move forward with a greater sense of direction and a better idea of what you should be aiming for, but all in all these are quite well done.

For the most part, you've paid a lot of attention to how you're achieving a smooth transition between levels of density, working your way from very heavily in shadow on the left side of your gradient, to quite sparse and bare on the right. In some cases you didn't push all the way to full white on the right side, but you weren't getting stuck in any one level of density based on the overuse of outlines - so this just tells me that you may have forgotten to have the far right side represent the other extreme, compared to the far left being a solid black bar.

That said, I am noticing in some of these textures, a reliance on the use of hatching lines where hatching itself is not inherently a part of the texture itself. It's not in all of them, and there are many more where you properly utilize clearly designed shadow shapes, but it is easy in some cases to fall back on hatching to give you something to transition between when struggling to find a reason to pile more ink on the right side of your texture.

There are also some cases where you altered the nature of the texture itself - like with your cactus row - packing in more little buds along the left side than you did on the left, basically making the texture itself more densely packed rather than the ink itself.

The reason we need to be careful about utilizing hatching in our textures is because hatching is generally used as an way to add form shading to objects without actually having to really dig into what textures are present. It's something generic and all-purpose that doesn't necessarily convey anything about the surface itself, but can allow us to capture how a surface may gradually get darker as it turns away from the light source. What we're doing by creating these texture gradients is using the information actually present on the object we're drawing to create a more meaningful replacement for hatching lines. Something that can do the same job, while serving more than just a decorative purpose - something that can communicate aspects of the object to the viewer. Something that can tell the viewer what it would feel like to run their fingers along it, instead of just conveying the same generic matte surface.

Now, that isn't to say that there aren't places where hatching is appropriate - it's just that when you choose to add hatching lines, you need to make the case for that very strongly for yourself. For example, if we look at your wood texture on this page, the study on the left side shows very clearly that the texture is driven by these little strips of form that are staggered and layered at different depths, creating the impression of a very rough surface. There is however a minimum "width" for these strips which they don't generally shrink beyond.

Using hatching in your gradient however implies that the strips are going to get infinitely thin as the texture gets more and more dense. This is similar to the cactus - for the purposes of controlling the density of ink/linework, you're changing the nature of the texture that is present. Instead of using hatching to create more strips, what we would do here is take the individual shadows already cast by the strips that exist within our texture, and simply expand them. Instead of more lines, those lines get thicker, until the far left side gets engulfed in black ink.

This also would create a more consistent shift to that full-black bar on the left, which often still featured a small but noticeable jump when moving into the texture. The real goal here is not to be able to pin down exactly where that bar ends, and where the texture itself begins.

The last thing I want to mention has to do with the concern you raised about indicating contours, capturing concave forms, etc. My response won't actually answer your question, but rather underline a key misunderstanding that the question presents. As discussed in these notes, we're not at any point actually drawing the textural forms themselves. Drawing forms explicitly is reserved for the construction itself, the forms onto which we eventually apply texture. When drawing texture, we instead always rely on implying the presence of those forms by drawing around them, by capturing the impact they have on their surroundings. While this does mean that we're not applying any contour lines, or going to any lengths to make them appear rounded, convex, or whatever else - it's because it's generally not necessary. At this scale and with the sheer number of forms present, the viewer's generally able to pick up the gist of what we're putting down, as far as they need to. Again, remember - we're giving the impression of what it would feel like to run your fingers over the surface. That's all we need to achieve - a sensation in the back of their minds.

So, for cases like the crocodile skin, no attempt should have been made to shade those forms. All we focus on are the shadows those forms cast. These cast shadows actually establish the relationship between the form casting the shadow, and the surface upon which it's cast. If you end up with a longer shadow, this usually means that the form in question is quite a bit taller than the surface. If the cast shadow is minimal, barely a sliver, then it means the form is negligible - which also means its relevance to the question of "how would it feel to run my fingers across this surface" would also be virtually negligible, just producing the slightest bump.

My point here is that the focus is not at all about being extremely accurate in the specifics of what we're capturing, but rather on the tactile impression it has on the viewer. Nothing more than that.

So! I've laid out quite a bit, but again - your work here really is well done. You've proven yourself to have very sharp observational skills throughout, and there are loads of textures here where you nave largely focused on implying the presence of those forms, rather than attempting to draw them explicitly. I'll go ahead and mark this challenge as complete. Congratulations.