Hello huong, I will be happy to give you some critique. Here is what I can say:

When is comes to drawing the detail on the leaves exercise, remember that the way to add texture is by drawing the shadow shapes and not the outlines of the texture, just like you went over in lesson 2. This video from lesson 2 explains this concept better: https://www.youtube.com/watch?v=sueXWvNZ0ss&ab_channel=Uncomfortable. You also made this mistake on the mushroom exercise, where the forms are first outlined, and then you draw the shadow shapes. Instead, draw only the shadow shapes and not the outlines of the forms.

For the branches exercise, try your best to elimanate the tailing that occours from ellipse to ellipse. In order to do this, you will really have to consider how the curve should connect to not only the ellipse next, but how it should lead in to the ellipse after that one. It takes practice. Even if the line deviates, you should start the next line where the line that deviates left off instead of trying to correct it.

The next thing I want to mention is your cast shadows. Your cast shadows are pretty solid, but there are a few things here and there that I can help with. The purpose of cast shadows is to help forms appear like they are "on top" of other forms, and well drawn cast shadows can help the 3d effect of a drawing.

A drawing that does not have good cast shadows is the cactus- there would be nothing to cast those shadows, and they only add to the high contrast of the image. The version without shadows looks much better. Another drawing with confusing cast shadows is the aloe/succulent plant, where it looks like you added shadows to any part of the plant that "dipped" in on itself, instead of adding shadows that were cast by the other parts of the plant.

One of the drawings with decent cast shadows would be the one with the white petals, where the shadows are pretty consistent and "stick" to the forms they are being cast on, like in this image: https://d15v304a6xpq4b.cloudfront.net/lesson_images/516f8d4f.jpg.

They key to cast shadows is knowing when to use them and making sure they "stick" to whatever form they are being cast on. Don't overdo them like in the cactus, the aloe or like you did in the white petal plant. If you need to communicate to the viewer that a petal/object is on top of another, the cast shadow will help.

Since it seems like you want to get better at this, I will try to assign you exercises to help you out. Draw 2 more plants of your choice. Detail is not required, but use cast shadows. Make they are only used when forms are overlapping, that they are used to differentate forms, and that they "stick" to the shape of the forms under them. Don't overdo them. Post the reference image as well.