Thick segments of tissue would not be enough for Annie Dillard's needs.
Explanation:
When Annie refers to "tissue," she is referring to biological tissues that will be used in the new microscope kit that she received from her parents. For it to be possible for her to visualize biological tissues in this microscope, it is necessary that she has "thin segments of fragile tissue," because thick segments of tissue would not allow a good visualization, with details, in her microscope.
