Schleiden and Schwann are credited with proposing the idea that all plant life (Schleiden) and animal life (Schwann) are composed of cells, or are the product of cells, leading to the discovery of governing laws which are shared by cells from both forms of life. Although credited with this discovery, both scientists built upon foundations laid by previous research into the their respective fields.
Basic research into cell nuclei was confirmed in 1835, and it was from this basis that both Schleiden and Schwann began their inquiries.
Despite proposing that all plant matter was composed directly of cells or cell products, Schleiden was initially mistaken on a number of other fronts. In particular, he continued to champion the idea that cells were created from a crystallization process, despite the rejection of this idea some years earlier by other scientists.
Schwann could arguably be said to have made the greater contribution to science, as far less was known about cell role and reproduction in animals. Schwann took the same position as Schleiden, in that all animals were composed of cells or cell products, but also went further in asserting that the governing laws of cells was the same in both plant and animal life.
Although some of Schleiden's ideas were wrong, the impact of his initial research should not be underestimated, as it changed the focus of scientific inquiry at the time, which led to a much greater understanding of cells and their role.