Cytology is the branch of biology dealing with the study of cells, their structure, function, biochemistry, etc. The discipline began with Robert Hooke's microscopic studies of cork in 1665, and the various forms of microscope are the main tools of cytology. An often used technique is tissue culture. In the 19th century, a cell theory was developed which suggested that cells are the basic units of organisms. Recent cytological study has focused on the chemistry of cell components (cytochemistry). See also histology.