Starting with your arrows, you're doing a good job of drawing these with confident, flowing linework, and I can definitely see clear efforts being made to have the gaps between your zigzagging sections compress as they move farther back in space to better show the depth of the scene.

That confident flow translates fairly well into your leaves exercises, where I think you're doing a pretty good job of capturing not only how those leaves sit statically in space, but also how they move through that space. You're also doing a good job in most cases of adhering to the simpler structure of your leaves when building up further edge detail. There's only one - this one where you're being a little more loose with the decisions made in previous phases. With the original leaf form you put down (which would ultimately contain all the smaller individual ones), you were establishing how far out each smaller leaf would extend. With that decision made, choosing to change that decision after the fact by having them extend out further would introduce contradictions into the construction, which in turn would undermine the illusion of solidity and structure. Once a decision has been made - even if it doesn't reflect the reference perfectly - you'll still have to adhere to it.

Moving onto your branches, don't forget that you should be drawing through each and every ellipse you draw freehand for these lessons. Aside from that, you're mostly doing a pretty good job with this exercise. There are a few places where you don't quite extend your edge segments fully halfway towards the next ellipse, leaving yourself with less to use as a runway for the next segment, but all in all you're doing a good job of getting them to flow seamlessly into one another, and are doing pretty well at maintaining a relatively consistent width across the entirety of a branch's length.

Looking at your plant constructions, there are a number of things you're doing a pretty good job with, as well as some issues that come up fairly frequently throughout the entire set. The issue that stands out most to me is that you have a tendency to put down your construction confidently, then come back over to trace your linework with a darker stroke. The key word here is trace. Set aside the obvious definition of the word, and focus on what it really means. Going back over a drawing slowly and steadily, following the lines as they exist on the page itself. It's a process that causes us to think about the drawing itself as a two dimensional arrangement of marks, and in doing so, the result is often one that comes out look flat. For example if you look here, you focused on reinforcing the 2D silhouette of all those smaller petals together, instead of using line weight to clarify how one petal might overlap another. As a whole, tracing over the existing lines in this way flattened out a lot of your drawings, reminding the viewer in a subtle, subconscious fashion, that what they're looking at is just lines and shapes on a flat page.

The solution to this has two parts - firstly, don't overuse line weight. Only concentrate it where you want to clarify a specific overlap between forms, and understand that line weight does not have to continue along the entirety of a silhouette or cover a whole line. Generally line weight should be added in very limited sections, and blended back into your stroke. When drawing lines with confidence, it usually results in tapering towards the start end end of the stroke, which helps it to both appear more lively and energetic, while also making it possible to blend it back into an existing line.

That leads to the second part - every single mark you draw should be drawn with the ghosting method, including the addition of line weight. Focus on your lines as they exist as edges in 3D space, not as lines on your page, always thinking about how they flow through three dimensions.

The potted plant on the left side of this page demonstrates a far better use of line weight, where it's much more subtle, and the linework as a whole does appear to be more confident and more focused on always reinforcing the fact that things are 3D.

Another thing that can quickly make a drawing appear flat is when we treat the forms we've drawn as though they are in fact just flat shapes. For example, looking at the cactus on this page, I can see how you started out with a larger ball form, then when adding the more complex aspects of the cactus' ridges, you cut back into the silhouette of that initial mass. The thing is, you cut back into it as it exists in two dimensions on the page, neglecting to treat it as a 3D form. A better way to approach this would be to start with a smaller ball form and then building the cactus' ridges on top of it, building additional forms upon the existing structure rather than cutting back. This is referred to as 'additive construction'. There is a correct way to approach 'subtractive' construction (where we cut into forms) as shown here, but this does not work as effectively for organic objects. Whenever possible, always deign to use additive construction. While yielding better results, it'll also further develop your understanding of those forms in three dimensions.

Now, despite the major issue I called out (in regards to tracing when adding line weight), you are otherwise demonstrating a pretty good grasp of construction as applied here, and I think it's really just the last step that throws many of your drawings off. As such, I think you're ready to consider this lesson as complete and to move on. Just be sure to work on your application of line weight, and prioritize the illusion that the things you're looking at are 3D at all times. Lesson 4 will give you ample opportunity to continue practicing this.