He would have came to the conclusion that the cells were that of a plant because plant cells are rectangular (it has a normal shape) and it has a central vacuole to store food and water, it would also contain a cell wall and a cell membrane. I think that Dimitri is correct if he saw all these aspects. Plant cells contain an organelle called chloroplast, this organelle has chlorophyll which supports photosynthesis meaning that plant cells can make their own food while other cells cannot.