If a cambium cell is to become bark or wood, its membrane grows into the vacuole and lays out additional amounts of cell wall, thus gaining stiffness. If the cambium cell is to become part of a vascular bundle for transmitting sap, it becomes cylindrical and develops openings at each end that pass sap from cell to cell. The membranes of animal cells contain a flexible protein material that provides protection yet permits the cells to change in size and shape. This flexibility makes it possible for animals to move about. Most many-celled animals need structures such as bones to support their bodies and shells or tough skins to protect them.
Special cells secrete the substances needed to form supportive or protective tissue.
History of the Cell Theory Cells were first described by the English scientist Robert Hooke, who in 1665 published a book about his findings. Hooke had sliced off thin sections of cork. With a microscope of his own design he was able to see the minute, boxlike units of which the cork was made up. More detailed investigations became possible with the development of improved compound microscopes. The Scottish botanist Robert Brown discovered the cell nucleus in 1831.