In 1838, German botanist Matthias Schleiden found that all plant tissues are made up of cells which have been essentially similar structure. German zoologist Theodor Schwann in 1839 working independently, observed that animal cells do not have cell walls but are otherwise similar amongst themselves. He put forward cell hypothesis- bodies of animal and plants are made up of cells. Schleiden and Schwann in 1839 compared their findings and formulated cell theory. The two basic tenets of modern cell theory are:
1. All living organisms are composed of cells and their products.
2. All cells arise from preexisting cells.
Viruses are an exception to the cell theory because viruses are a non living crystallizable particles. The do not have cellular structure.