Franz Unger (1800-1870) - whose courses in plant physiology Mendel attended in Vienna - had raised questions about the determination of the number of different forms under which hybrids appeared, and the statistical relationships between these forms. In his Botanical Letters (1852), Unger denies the fixity of species, maintaining the variability of subspecies, and drawing attention to yet another new area of scientific exploration, namely cell transformation, the subject of the next section.
 Cell theory had only recently been formulated by a number of scientists, from the Czech J. E. Purkinje (1787-1869) (also famed for his study on the conductive tissue of the heart) who, in 1837, observed a parallel between the structure of plant and animal tissue; through the German botanist Matthias Schleiden (1804-1881)[i], who proposed that all plant tissue is composed of nucleated cells; to another German scientist, Theodor Schwann (1810-1882), who suggested "cellular formation" for both plant and animal tissue. Contrary to Schleiden, Unger showed that plants cells originate by division rather than by spontaneous formation. This would have been a crucial insight for Mendel.
Furthermore, Schleiden's and Unger's emphasis on the need for "sharp" experiments, the application of mathematical laws to biology and botany, as well as their innovative definition of cells, profoundly influenced Mendel's thinking and method.
 Mendel was indebted to Unger's work on the role and behaviour of cells in pollination, and his observation of the production of new varieties through cross fertilisation, in particular the notion that this involved the union of just two cells. Through his experiments with pea plants, Mendel further explored the question of how the union of two cells produce a new organism, the enigma of generation. He concentrated on what, until then, had remained overlooked: the importance of considering pairs of observable characters and the statistical laws governing the pattern of their re-appearance in the off-springs.
Naturally occurring hybrids were known to display characters belonging to both parents, as observed with respect to height and leaf distribution in the specimens of Primula from the 18th century Bobart's Herbarium. Applying the knowledge that he had gained through his study of experimental physics and mathematical sciences, Mendel developed special techniques of artificial pollination in order to control the crossing of species. Using these techniques he discovered that new patterns of combinations of characters would emerge in the hybrids if large enough numbers of plants were observed across several generations.
 Mendel's experiments with Pisum started in the summer of 1856; in 1859 Darwin's The Origin of Species was published. Mendel read the 1863 German translation, making a few pencil notes on the margins and at the back of book, as he did with another of Darwin's books, The Variation of Animals and Plants under Domestication (London,1868)[i]. Mendel seemed to accept the idea of the evolution of organic forms. His own understanding of plant hybrids was however at odds with Darwin's more traditional views. Crucially, Mendel focused on what is inherited (i.e. characters) and the patterns through which traits are transmitted from one generation to the next.
Within another of nature's realms, Mendel experimented with the crossing of bees, [i] a technique that was gaining importance among Moravian bee-keepers. In bee-crossing, he was looking for further confirmation of his earlier results with peas, and in the Abbey's garden a bee house was built for him, equipped with hives according to his own design.
Franz Unger
Matthias Schleiden
Bees