Congratulations on getting through all 25 of these! There are definitely some areas of notable strength and success, along with a number of issues I do want to call out.

To start, a lot of your initial studies - that is, those in the square on the far left of each row - came out really well and showed a lot of conscious, careful observation of not only your reference, but of the actual forms that are present there. The shadow shapes you've drawn here. These are especially strong towards the beginning - perhaps where you had more patience and time to give them, as towards the end you did seem a little burnt out. These on the first page show a really clear focus on the textural forms themselves, ensuring that each shadow is designed to imply each textural form's specific properties. It shows me that you understood what the reference image was depicting, rather than just copying it over.

Now, let's look into some areas where things can be improved upon:

  • The most significant shortcoming here was that your textural gradients all tend to have a pretty significant jump towards the end, where they hit the back bar on the far left. That black bar, as explained here, basically is there to establish the farthest extent this gradient is going to reach. The idea is that if we define solid black on the far left, and solid white on the far right, then the student will have to create a gradient that spans between these extents. In the end, the black bar should be fully integrated into your gradient, with the point where we move from the bar to the actual texture study should be completely obscured and unclear. We shouldn't be able to point out where the bar ends, and where the texture begins. That is something you missed out on these textures, and the extreme darks should have been pushed along much farther into your gradients.

  • If we jump almost to the end, there's a texture at the bottom of this page where you tackled something either like long hair or fur. In your gradient, you focused on the marks you put down as relating to each individual shock of hair or fur. As I've shown here, we actually want to be drawing that which falls in between the forms of the hair/fur itself. We're focusing on the shadows, after all, and they fall thickest where different forms meet (as mentioned in the diagram here).

  • The previous point comes up in other kinds of textures as well - for example, corn and even pebbles. If you look at the pebble texture at the top of this page, you actually focused on the areas between your pebbles when doing the original study (on the left side), but when you went about drawing the gradient itself, you shifted back to thinking about each individual pebble. As you pushed further to the right, you did shift back towards the correct approach, but as a whole you definitely need to focus more on the areas where your forms meet one another, rather than on trying to outline them all.

  • While bubbles definitely can be a texture, I don't think they're something I'd try to capture in a challenge like this, simply because of our tool limitations. Transparent forms don't really lend themselves too well to working in stark black and white as we do here.

  • One last thing - and this is minor, more of a suggestion - always try and make sure that when you draw your shadow shapes, that you're actually outlining them first, then filling them in. There will inevitably be cases where you feel tempted to just draw certain thinner strokes directly (drawing them as a single stroke, rather than outlining then filling them in), but as shown here there's an important difference between a shape that has been outlined, and a single stroke, in terms of their dynamism. Even though the former obviously can't get thinner than a certain threshold, it's actually perfectly okay to allow certain cracks and marks get so thin that they're not visible in the drawing. Not everything has to be drawn, and sometimes we have to allow some of those marks to get a little more lost.

So! I hope my feedback here has been helpful. I definitely see the capacity for strong observation, and all the right ingredients to transfer that into effective textural gradients, but there are some shortcomings to be sure. All of this can certainly be addressed, and it by no means undermines your accomplishment here. You can go ahead and consider this challenge complete - just be sure to apply what I've shared with you here to your own work as you move forwards.