William Nylander was a Finnish botanist and entomologist.
Nylander taught at the University of Helsinki

University of Helsinki

The University of Helsinki is a university located in Helsinki, Finland since 1829, but was founded in the city of Turku in 1640 as The Royal Academy of Turku, at that time part of the Swedish Empire. It is the oldest and largest university in Finland with the widest range of disciplines available...

 for a number of years before later moving to Paris, where he was to live until his death in 1899.
He was a very important figure in the field of lichenology

Lichenology

Lichenology is the branch of mycology that studies the lichens, symbiotic organisms made up of an intimate symbiotic association of a microscopic alga with a filamentous fungus....

.

Nylander pioneered the technique of determining the taxonomy of lichens by the use of chemical reagents, such as tinctures of iodine

Iodine

Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

 and hypochlorite

Hypochlorite

The hypochlorite ion, also known as chlorate anion is ClO−. A hypochlorite compound is a chemical compound containing this group, with chlorine in oxidation state +1.Hypochlorites are the salts of hypochlorous acid...

, still used by lichenologists to this day.

Nylander was also the first to realise the effect of atmospheric pollution on the growth of lichens, an important discovery that paved the way for the use of lichens to detect pollution and determine the cleanness of air.