Calvin carried out ‘feeding experiments’ where radioactive labelled carbon was fed to plants. 14CO2 is taken up the cells and is then fixed into photosynthesis. 14CO2 was induced into the plants and monitored. Samples of photosynthesising cells taken at frequent intervals after feeding contained a sequence of radioactively labelled intermediates and products of the photosynthetic pathway. They were then isolated by chromatography. Chlorella was used, instead of mesophyll tissue, because they are easy to sample and have identical photosynthesis pathways