1. The problem asks to identify the scientist who studied plants, used a microscope, and concluded that plants are made of cells in 1838. 2. This is related to the development of cell theory, which involved several scientists. 3. The key clues are the year 1838, the study of plants, and the conclusion that plants are made of cells. 4. Matthias Schleiden is the scientist known for these contributions: he studied plants, used a microscope, and concluded that plants are composed of cells in 1838. 5. Therefore, the answer is Schleiden.