In the 1600's, Robert Hooke, and English scientist, used a crude microscope to examine bits of cork. Cork is derived from the bark layer of certain trees. Upon seeing the cork under the microscope, Hooke named the spaces within the cork 'cells', because they looked like empty rooms of a monastery. Although he coined the term cell, an ironic feature of cork is that it is dead plant material. Knowing this, what feature of cells would Robert Hooke NOT have been able to observe under the microscope while looking at cork?