Gregor Mendel, Franz Unger, Carl Nägeli and the magic of numbers

This paper aims to illustrate the influence of Franz Unger’s (1800–1870) and Carl Wilhelm Nägeli’s (1817–1891) anatomical and developmental works on Gregor Mendel’s (1822–1884) use of numerical ratios in biological inquiry. All hypotheses concerning Mendel’s sources of inspiration have hitherto overlooked the cytological teaching of Unger, his professor of botany. In the 1830s and 1840s, he was a pioneer of cell theory. His publications, including his university textbooks, are characterised by a particulate and quantifying approach towards vital phenomena. Special attention will be paid to Unger’s and Nägeli’s ‘law of plant growth’, which shows some striking basic correspondences with Mendel’s approach. This demonstrates, firstly, that mathematical thinking was not alien to mid-nineteenth century botany, and secondly, that Mendel’s conceptions were influenced by an even greater variety of ideas circulating around 1850.