Around 1838, Matthias Schleiden discovered that all plant tissues he examined with a microscope were composed of cells. He was the first to make a general statement about them, declaring that all parts of plants were composed of cells. Further, he said that plant embryos grew from single cells. He was also an early evolutionist, hypothesizing about the possibility of large-scale evolution over time in plants.
Matthias Schleiden's discoveries were major steps in the formation of cell theory, although they weren't the first discoveries of cells. The first real discovery of plant cells, although not their significance, was by Robert Hooke in 1665. He discovered that pieces of cork were composed of a multitude of tiny hollow chambers. These were only the cell walls that were visible, because of the way cork cells die and hollow. In 1670 Anton van Leeuwenhoek observed and described single-celled organisms in pond water.
Theodor Schwann made the same conclusion about animals and cells in 1839 as Schleiden did about plants. His conclusion came from the observation of cell-like structures embedded in cartilage. He further formulated the idea that all organisms are composed primarily of cells, and that cells are the basic unit of all life. This was the first true statement of cell theory.