The motion is also observed in living organisms e.g. the cellular contents of filamentous aquatic algae, or as early microscopists observed - the particles suspended in the medium within living pollen grains. It's probably best to become familiar with the motion first in non-living material in order to more easily distinguish the motion from e.g. the movement of bacteria or cellular activity. He was studying the fertilisation process in the plant Clarkia pulchella when he noticed the motion in tiny particles suspended within the medium of living pollen grains. He went on to show that the motion occurred in non-living as well as living material. Some texbooks, even to university level, incorrectly state that he observed the movement of the pollen grains themselves.
Most pollen grains are too large to exhibit noticeable Brownian motion.
Although Robert Brown is best known for his study of the motion now named after him, as a botanist he also made other important contributions to science. It was not until later in the 19th century that correct physical explanations of the motion were formulated and Brownian motion was historically one of the first direct pieces of evidence for the existence of molecules. This fascinating paper describes how Robert Brown's observations were repeated using one of Brown's original microscopes. This demonstration was carried out to dispel the suggestion that Brown's microscopes were not good enough to show the motion. The paper also discusses many aspects of Brown's life and work.