Discovery of the Cell Nucleus
The term cell nucleus was used by Robert Brown for the first time in 1831 in a paper to the Linnean Society and it was published in 1833. At the time of publishing he did not realize that cell nucleus was present in cells other than those belonging to plants.
During his stay in Australia Brown studied nearly 1700 plant species and other notable observations include the identification of naked ovule of the gymnospermae.
While he was scrutinizing the epidermis of a collection of orchids with his microscope, Brown found that the cells contained an opaque spot. He used the term areola to describe them. They had been seen by other scientists previously who most likely regarded them as insignificant.
But Brown was not to ignore this specific observation and he further noted that the spot was not limited to the epidermis as it could also be observed during the early stage of pollen formation. He sensed that this spot was a key component of cells and called them "nucleus" - a term which is still being used today.
Another point worth mentioning at this stage is that though he is universally acknowledged as being the discoverer of the cell nucleus, Brown also gave credit to Franz Bauer who was a contemporary botanist and had made similar observations.
Apart from discovering the cell nucleus, Brown's other important contribution to science was the discovery of the phenomenon known as "Brownian motion." Looking at pollen grains suspended in water he could see that they were jiggling about, but he couldn't understand why. It was Einstein in 1905 who provided an explanation, namely that the pollen was moving because it was being hit by invisible water molecules.