Complex sentence: It was known that certain legumes could take up nitrogen from the air and fix it to the soil but it took the development of bacteriology towards the end of the 19th century to led to an understanding of the role played in nitrogen fixation by bacteria.
Split into simpler sentences: It was known that certain legumes could take up nitrogen from the air and fix it to the soil. The development of bacteriology towards the end of the 19th century led to an understanding of the role played in nitrogen fixation by bacteria.