Question

how did robert brown know that his explanation was wrong

Answer 1

Robert Brown (1773-1858) was a distinguished microscopist and botanist. He discovered the naked ovule of the gymnospermae, a most exacting piece of microscopical investigation, and carefully documented the phenomenon now known as Brownian Movement. The idea that he did not observe this physical phenomenon has often surfaced in the past. It has been said that the optical capacity of his simple microscopes was too limiting, and many modern accounts erroneously state that Brown observed the movement of pollen grains. That is not correct: most are several orders of magnitude too large to exhibit the phenomenon.

In 1831, while investigating the fertilization mechanisms of plants in the Orchidaceae and Asclepiadaceae families, he noted the existence of a structure within the cells of orchids, as well as many other plants, that he termed the “nucleus” of the cell. Although his were not the first observations of cell nuclei, his designation of the term has persisted. His observations testify to the range and depth of his pioneering microscopical work and his ability to draw far-reaching conclusions from isolated data or selected structures.

Brown was elected a fellow of the Royal Society in 1810 and served as president of the Linnean Society from 1849 to 1853. A number of Australian plant species, including Brown’s banksia (Banksia brownii) and Brown’s box (Eucalyptus brownii), are named after him.
This went him wrong.
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Answer 2

Answer:Robert Brown knew his explanation was wrong when he put some dust particles in water and he knew that dust particles were not alive but they moved in the same way the pollen grains did.