Final answer:
Matthias Schleiden studied plant tissues, focusing on plant cells, and contributed to the unified cell theory, recognizing the cell as the fundamental unit of plant structure.
Step-by-step explanation:
Matthias Schleiden, a German botanist, studied plant tissues by making extensive microscopic observations. In particular, Schleiden focused on plant cells, and he was one of the first to recognize them as the fundamental unit of plant structure.
Schleiden's research led to his contribution to the unified cell theory, which he proposed with Theodor Schwann, stating that all living things are composed of cells, the cell is the basic unit of life, and all new cells arise from existing cells. Schleiden's work was important because visualizing plant cells was relatively straightforward thanks to their thick cell walls, which were clearly visible under a microscope.